diff --git a/iamf/cli/proto_to_obu/tests/BUILD b/iamf/cli/proto_to_obu/tests/BUILD
index a00b0c7..3ea844e 100644
--- a/iamf/cli/proto_to_obu/tests/BUILD
+++ b/iamf/cli/proto_to_obu/tests/BUILD
@@ -148,6 +148,7 @@ cc_test(
         "//iamf/obu:types",
         "@com_google_absl//absl/container:flat_hash_map",
         "@com_google_absl//absl/status:status_matchers",
+        "@com_google_absl//absl/types:span",
         "@com_google_googletest//:gtest_main",
         "@com_google_protobuf//:protobuf",
     ],
diff --git a/iamf/cli/proto_to_obu/tests/parameter_block_generator_test.cc b/iamf/cli/proto_to_obu/tests/parameter_block_generator_test.cc
index 83b0332..cc8c110 100644
--- a/iamf/cli/proto_to_obu/tests/parameter_block_generator_test.cc
+++ b/iamf/cli/proto_to_obu/tests/parameter_block_generator_test.cc
@@ -11,6 +11,7 @@
  */
 #include "iamf/cli/proto_to_obu/parameter_block_generator.h"
 
+#include <array>
 #include <cstdint>
 #include <list>
 #include <memory>
@@ -18,6 +19,7 @@
 
 #include "absl/container/flat_hash_map.h"
 #include "absl/status/status_matchers.h"
+#include "absl/types/span.h"
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
 #include "iamf/cli/audio_element_with_data.h"
@@ -44,11 +46,12 @@ using ::absl_testing::IsOk;
 
 constexpr DecodedUleb128 kCodecConfigId = 200;
 constexpr DecodedUleb128 kAudioElementId = 300;
-constexpr DecodedUleb128 kMixPresentationId = 1337;
 constexpr DecodedUleb128 kParameterId = 100;
 constexpr DecodedUleb128 kParameterRate = 48000;
 constexpr DecodedUleb128 kDuration = 8;
 constexpr bool kOverrideComputedReconGains = false;
+constexpr std::array<DecodedUleb128, 1> kOneSubstreamId{0};
+constexpr std::array<DecodedUleb128, 4> kFourSubtreamIds{0, 1, 2, 3};
 
 TEST(ParameterBlockGeneratorTest, NoParameterBlocks) {
   absl::flat_hash_map<DecodedUleb128, PerIdParameterMetadata>
@@ -109,7 +112,7 @@ void ConfigureDemixingParameterBlocks(
 }
 
 void InitializePrerequisiteObus(
-    const std::vector<DecodedUleb128>& substream_ids,
+    absl::Span<const DecodedUleb128> substream_ids,
     absl::flat_hash_map<DecodedUleb128, CodecConfigObu>& codec_config_obus,
     absl::flat_hash_map<DecodedUleb128, AudioElementWithData>& audio_elements) {
   constexpr uint32_t kSampleRate = 48000;
@@ -153,7 +156,7 @@ TEST(ParameterBlockGeneratorTest, GenerateTwoDemixingParameterBlocks) {
   // Initialize pre-requisite OBUs.
   absl::flat_hash_map<DecodedUleb128, CodecConfigObu> codec_config_obus;
   absl::flat_hash_map<DecodedUleb128, AudioElementWithData> audio_elements;
-  InitializePrerequisiteObus(/*substream_ids=*/{0}, codec_config_obus,
+  InitializePrerequisiteObus(kOneSubstreamId, codec_config_obus,
                              audio_elements);
 
   // Add a demixing parameter definition inside the Audio Element OBU.
@@ -268,7 +271,7 @@ TEST(ParameterBlockGeneratorTest, GenerateMixGainParameterBlocks) {
   // Initialize pre-requisite OBUs.
   absl::flat_hash_map<DecodedUleb128, CodecConfigObu> codec_config_obus;
   absl::flat_hash_map<DecodedUleb128, AudioElementWithData> audio_elements;
-  InitializePrerequisiteObus(/*substream_ids=*/{0}, codec_config_obus,
+  InitializePrerequisiteObus(kOneSubstreamId, codec_config_obus,
                              audio_elements);
 
   // Add param definition. It would normally be owned by a Mix Presentation OBU.
@@ -428,7 +431,7 @@ TEST(ParameterBlockGeneratorTest, GenerateReconGainParameterBlocks) {
   // Initialize pre-requisite OBUs.
   absl::flat_hash_map<DecodedUleb128, CodecConfigObu> codec_config_obus;
   absl::flat_hash_map<DecodedUleb128, AudioElementWithData> audio_elements;
-  InitializePrerequisiteObus(/*substream_ids=*/{0, 1, 2, 3}, codec_config_obus,
+  InitializePrerequisiteObus(kFourSubtreamIds, codec_config_obus,
                              audio_elements);
 
   // Extra data needed to compute recon gain.
@@ -486,7 +489,7 @@ TEST(Initialize, FailsWhenThereAreStrayParameterBlocks) {
   ConfigureDemixingParameterBlocks(user_metadata);
   absl::flat_hash_map<DecodedUleb128, CodecConfigObu> codec_config_obus;
   absl::flat_hash_map<DecodedUleb128, AudioElementWithData> audio_elements;
-  InitializePrerequisiteObus(/*substream_ids=*/{0, 1, 2, 3}, codec_config_obus,
+  InitializePrerequisiteObus(kFourSubtreamIds, codec_config_obus,
                              audio_elements);
 
   // Construct and initialize.
diff --git a/iamf/cli/tests/cli_test_utils.cc b/iamf/cli/tests/cli_test_utils.cc
index a0e687c..0f2116d 100644
--- a/iamf/cli/tests/cli_test_utils.cc
+++ b/iamf/cli/tests/cli_test_utils.cc
@@ -174,7 +174,7 @@ void AddAacCodecConfigWithId(
 
 void AddAmbisonicsMonoAudioElementWithSubstreamIds(
     DecodedUleb128 audio_element_id, uint32_t codec_config_id,
-    const std::vector<DecodedUleb128>& substream_ids,
+    absl::Span<const DecodedUleb128> substream_ids,
     const absl::flat_hash_map<uint32_t, CodecConfigObu>& codec_config_obus,
     absl::flat_hash_map<DecodedUleb128, AudioElementWithData>& audio_elements) {
   // Check the `codec_config_id` is known and this is a new
@@ -187,10 +187,9 @@ void AddAmbisonicsMonoAudioElementWithSubstreamIds(
   AudioElementObu obu = AudioElementObu(
       ObuHeader(), audio_element_id, AudioElementObu::kAudioElementSceneBased,
       0, codec_config_id);
-  obu.audio_substream_ids_ = substream_ids;
   obu.InitializeParams(0);
   obu.InitializeAudioSubstreams(substream_ids.size());
-  obu.audio_substream_ids_ = substream_ids;
+  obu.audio_substream_ids_.assign(substream_ids.begin(), substream_ids.end());
 
   // Initialize to n-th order ambisonics. Choose the lowest order that can fit
   // all `substream_ids`. This may result in mixed-order ambisonics.
@@ -226,7 +225,7 @@ void AddAmbisonicsMonoAudioElementWithSubstreamIds(
 // Adds a scalable Audio Element OBU based on the input arguments.
 void AddScalableAudioElementWithSubstreamIds(
     DecodedUleb128 audio_element_id, uint32_t codec_config_id,
-    const std::vector<DecodedUleb128>& substream_ids,
+    absl::Span<const DecodedUleb128> substream_ids,
     const absl::flat_hash_map<uint32_t, CodecConfigObu>& codec_config_obus,
     absl::flat_hash_map<DecodedUleb128, AudioElementWithData>& audio_elements) {
   // Check the `codec_config_id` is known and this is a new
@@ -239,7 +238,8 @@ void AddScalableAudioElementWithSubstreamIds(
   AudioElementObu obu(ObuHeader(), audio_element_id,
                       AudioElementObu::kAudioElementChannelBased, 0,
                       codec_config_id);
-  obu.audio_substream_ids_ = substream_ids;
+  obu.InitializeAudioSubstreams(substream_ids.size());
+  obu.audio_substream_ids_.assign(substream_ids.begin(), substream_ids.end());
   obu.InitializeParams(0);
 
   EXPECT_THAT(obu.InitializeScalableChannelLayout(1, 0), IsOk());
diff --git a/iamf/cli/tests/cli_test_utils.h b/iamf/cli/tests/cli_test_utils.h
index 57a2aab..5c1fefe 100644
--- a/iamf/cli/tests/cli_test_utils.h
+++ b/iamf/cli/tests/cli_test_utils.h
@@ -19,7 +19,6 @@
 #include <vector>
 
 #include "absl/container/flat_hash_map.h"
-#include "absl/status/statusor.h"
 #include "absl/strings/string_view.h"
 #include "absl/types/span.h"
 #include "iamf/cli/audio_element_with_data.h"
@@ -92,7 +91,7 @@ void AddAacCodecConfigWithId(
  */
 void AddAmbisonicsMonoAudioElementWithSubstreamIds(
     DecodedUleb128 audio_element_id, uint32_t codec_config_id,
-    const std::vector<DecodedUleb128>& substream_ids,
+    absl::Span<const DecodedUleb128> substream_ids,
     const absl::flat_hash_map<uint32_t, CodecConfigObu>& codec_config_obus,
     absl::flat_hash_map<DecodedUleb128, AudioElementWithData>& audio_elements);
 
@@ -106,7 +105,7 @@ void AddAmbisonicsMonoAudioElementWithSubstreamIds(
  */
 void AddScalableAudioElementWithSubstreamIds(
     DecodedUleb128 audio_element_id, uint32_t codec_config_id,
-    const std::vector<DecodedUleb128>& substream_ids,
+    absl::Span<const DecodedUleb128> substream_ids,
     const absl::flat_hash_map<uint32_t, CodecConfigObu>& codec_config_obus,
     absl::flat_hash_map<DecodedUleb128, AudioElementWithData>& audio_elements);
 
diff --git a/iamf/cli/tests/cli_util_test.cc b/iamf/cli/tests/cli_util_test.cc
index 4deaa76..e5c69fc 100644
--- a/iamf/cli/tests/cli_util_test.cc
+++ b/iamf/cli/tests/cli_util_test.cc
@@ -11,6 +11,7 @@
  */
 #include "iamf/cli/cli_util.h"
 
+#include <array>
 #include <cstddef>
 #include <cstdint>
 #include <list>
@@ -55,6 +56,8 @@ constexpr DecodedUleb128 kParameterId = 99999;
 constexpr DecodedUleb128 kParameterRate = 48000;
 constexpr DecodedUleb128 kFirstSubstreamId = 31;
 constexpr DecodedUleb128 kSecondSubstreamId = 32;
+constexpr std::array<DecodedUleb128, 1> kZerothOrderAmbisonicsSubstreamId{
+    kFirstSubstreamId};
 
 TEST(WritePcmFrameToBuffer, ResizesOutputBuffer) {
   const size_t kExpectedSize = 12;  // 3 bytes per sample * 4 samples.
@@ -525,8 +528,8 @@ TEST(CollectAndValidateParamDefinitions,
   const std::list<MixPresentationObu> kNoMixPresentationObus = {};
   absl::flat_hash_map<DecodedUleb128, AudioElementWithData> audio_elements;
   AddAmbisonicsMonoAudioElementWithSubstreamIds(
-      kAudioElementId, kCodecConfigId, {kFirstSubstreamId}, input_codec_configs,
-      audio_elements);
+      kAudioElementId, kCodecConfigId, kZerothOrderAmbisonicsSubstreamId,
+      input_codec_configs, audio_elements);
   auto& audio_element = audio_elements.at(kAudioElementId);
   audio_element.obu.InitializeParams(1);
   audio_element.obu.audio_element_params_[0] = AudioElementParam{
@@ -547,8 +550,8 @@ TEST(CollectAndValidateParamDefinitions,
   const std::list<MixPresentationObu> kNoMixPresentationObus = {};
   absl::flat_hash_map<DecodedUleb128, AudioElementWithData> audio_elements;
   AddAmbisonicsMonoAudioElementWithSubstreamIds(
-      kAudioElementId, kCodecConfigId, {kFirstSubstreamId}, input_codec_configs,
-      audio_elements);
+      kAudioElementId, kCodecConfigId, kZerothOrderAmbisonicsSubstreamId,
+      input_codec_configs, audio_elements);
 
   // Add an extension param definition to the audio element. It is not possible
   // to determine the ID to store it or to use further processing.
diff --git a/iamf/cli/tests/demixing_module_test.cc b/iamf/cli/tests/demixing_module_test.cc
index 534b363..405a293 100644
--- a/iamf/cli/tests/demixing_module_test.cc
+++ b/iamf/cli/tests/demixing_module_test.cc
@@ -273,12 +273,13 @@ TEST(InitializeForReconstruction, CreatesNoDemixersForSingleLayerChannelBased) {
 
 TEST(InitializeForReconstruction, CreatesNoDemixersForAmbisonics) {
   const DecodedUleb128 kCodecConfigId = 0;
+  constexpr std::array<DecodedUleb128, 4> kAmbisonicsSubstreamIds{0, 1, 2, 3};
   absl::flat_hash_map<DecodedUleb128, CodecConfigObu> codec_configs;
   AddLpcmCodecConfigWithIdAndSampleRate(kCodecConfigId, 48000, codec_configs);
   absl::flat_hash_map<DecodedUleb128, AudioElementWithData> audio_elements;
   AddAmbisonicsMonoAudioElementWithSubstreamIds(kAudioElementId, kCodecConfigId,
-                                                {0, 1, 2, 3}, codec_configs,
-                                                audio_elements);
+                                                kAmbisonicsSubstreamIds,
+                                                codec_configs, audio_elements);
   DemixingModule demixing_module;
   EXPECT_THAT(demixing_module.InitializeForReconstruction(audio_elements),
               IsOk());
diff --git a/iamf/cli/tests/global_timing_module_test.cc b/iamf/cli/tests/global_timing_module_test.cc
index 0e55ac7..b2e0334 100644
--- a/iamf/cli/tests/global_timing_module_test.cc
+++ b/iamf/cli/tests/global_timing_module_test.cc
@@ -14,7 +14,6 @@
 #include <cstdint>
 #include <memory>
 #include <utility>
-#include <vector>
 
 #include "absl/container/flat_hash_map.h"
 #include "absl/status/status.h"
@@ -39,7 +38,6 @@ const DecodedUleb128 kSampleRate = 48000;
 const DecodedUleb128 kFirstAudioElementId = 0;
 const DecodedUleb128 kFirstAudioFrameId = 1000;
 const DecodedUleb128 kFirstParameterId = 0;
-const DecodedUleb128 kParameterIdForLoggingPurposes = kFirstParameterId;
 
 class GlobalTimingModuleTest : public ::testing::Test {
  protected:
@@ -47,8 +45,8 @@ class GlobalTimingModuleTest : public ::testing::Test {
     AddLpcmCodecConfigWithIdAndSampleRate(kCodecConfigId, kSampleRate,
                                           codec_config_obus_);
     AddAmbisonicsMonoAudioElementWithSubstreamIds(
-        kFirstAudioElementId, kCodecConfigId, {1000}, codec_config_obus_,
-        audio_elements_);
+        kFirstAudioElementId, kCodecConfigId, {kFirstAudioFrameId},
+        codec_config_obus_, audio_elements_);
     EXPECT_THAT(Initialize(), IsOk());
 
     TestGetNextAudioFrameStamps(kFirstAudioFrameId, 512, 0, 512);
@@ -128,14 +126,15 @@ TEST_F(GlobalTimingModuleTest, OneSubstream) {
 }
 
 TEST_F(GlobalTimingModuleTest, InvalidUnknownSubstreamId) {
+  constexpr DecodedUleb128 kSubstreamId = 9999;
+  constexpr DecodedUleb128 kUnknownSubstreamId = 10000;
   AddLpcmCodecConfigWithIdAndSampleRate(kCodecConfigId, kSampleRate,
                                         codec_config_obus_);
   AddAmbisonicsMonoAudioElementWithSubstreamIds(
-      kFirstAudioElementId, kCodecConfigId, {0}, codec_config_obus_,
+      kFirstAudioElementId, kCodecConfigId, {kSubstreamId}, codec_config_obus_,
       audio_elements_);
   EXPECT_THAT(Initialize(), IsOk());
 
-  const DecodedUleb128 kUnknownSubstreamId = 9999;
   TestGetNextAudioFrameStamps(kUnknownSubstreamId, 128, 0, 128,
                               absl::StatusCode::kInvalidArgument);
 }
@@ -152,22 +151,24 @@ TEST_F(GlobalTimingModuleTest, InvalidDuplicateSubstreamIds) {
 }
 
 TEST_F(GlobalTimingModuleTest, TwoAudioElements) {
+  constexpr DecodedUleb128 kFirstSubstreamId = kFirstAudioFrameId;
+  constexpr DecodedUleb128 kSecondSubstreamId = 2000;
   AddLpcmCodecConfigWithIdAndSampleRate(kCodecConfigId, kSampleRate,
                                         codec_config_obus_);
   AddAmbisonicsMonoAudioElementWithSubstreamIds(
-      kFirstAudioElementId, kCodecConfigId, {kFirstAudioFrameId},
+      kFirstAudioElementId, kCodecConfigId, {kFirstSubstreamId},
       codec_config_obus_, audio_elements_);
   const DecodedUleb128 kSecondAudioElementId = 1;
   ASSERT_NE(kFirstAudioElementId, kSecondAudioElementId);
   AddAmbisonicsMonoAudioElementWithSubstreamIds(
-      kSecondAudioElementId, kCodecConfigId, {2000}, codec_config_obus_,
-      audio_elements_);
+      kSecondAudioElementId, kCodecConfigId, {kSecondSubstreamId},
+      codec_config_obus_, audio_elements_);
   EXPECT_THAT(Initialize(), IsOk());
 
   // All subtreams have separate time keeping functionality.
-  TestGetNextAudioFrameStamps(kFirstAudioFrameId, 128, 0, 128);
-  TestGetNextAudioFrameStamps(kFirstAudioFrameId, 128, 128, 256);
-  TestGetNextAudioFrameStamps(kFirstAudioFrameId, 128, 256, 384);
+  TestGetNextAudioFrameStamps(kFirstSubstreamId, 128, 0, 128);
+  TestGetNextAudioFrameStamps(kFirstSubstreamId, 128, 128, 256);
+  TestGetNextAudioFrameStamps(kFirstSubstreamId, 128, 256, 384);
 
   TestGetNextAudioFrameStamps(2000, 256, 0, 256);
   TestGetNextAudioFrameStamps(2000, 256, 256, 512);
diff --git a/iamf/cli/tests/parameters_manager_test.cc b/iamf/cli/tests/parameters_manager_test.cc
index 30f8842..c37e435 100644
--- a/iamf/cli/tests/parameters_manager_test.cc
+++ b/iamf/cli/tests/parameters_manager_test.cc
@@ -41,6 +41,8 @@ using ::absl_testing::IsOk;
 constexpr DecodedUleb128 kCodecConfigId = 1450;
 constexpr DecodedUleb128 kSampleRate = 16000;
 constexpr DecodedUleb128 kAudioElementId = 157;
+constexpr DecodedUleb128 kFirstSubstreamId = 0;
+constexpr DecodedUleb128 kSecondSubstreamId = 1;
 constexpr DecodedUleb128 kParameterId = 995;
 constexpr DecodedUleb128 kSecondParameterId = 996;
 constexpr DecodedUleb128 kDuration = 8;
@@ -112,8 +114,8 @@ class ParametersManagerTest : public testing::Test {
     AddLpcmCodecConfigWithIdAndSampleRate(kCodecConfigId, kSampleRate,
                                           codec_config_obus_);
     AddAmbisonicsMonoAudioElementWithSubstreamIds(
-        kAudioElementId, kCodecConfigId,
-        /*substream_ids=*/{100}, codec_config_obus_, audio_elements_);
+        kAudioElementId, kCodecConfigId, {kFirstSubstreamId},
+        codec_config_obus_, audio_elements_);
 
     auto& audio_element_obu = audio_elements_.at(kAudioElementId).obu;
     AddDemixingParamDefinition(kParameterId, kSampleRate, kDuration,
@@ -554,8 +556,8 @@ TEST_F(ParametersManagerTest,
   // Add a second audio element sharing the same demixing parameter.
   constexpr DecodedUleb128 kAudioElementId2 = kAudioElementId + 1;
   AddAmbisonicsMonoAudioElementWithSubstreamIds(
-      kAudioElementId2, kCodecConfigId,
-      /*substream_ids=*/{200}, codec_config_obus_, audio_elements_);
+      kAudioElementId2, kCodecConfigId, {kSecondSubstreamId},
+      codec_config_obus_, audio_elements_);
   auto& second_audio_element_obu = audio_elements_.at(kAudioElementId2).obu;
   AddDemixingParamDefinition(kParameterId, kSampleRate, kDuration,
                              second_audio_element_obu,
diff --git a/iamf/cli/tests/profile_filter_test.cc b/iamf/cli/tests/profile_filter_test.cc
index 0782f18..8154681 100644
--- a/iamf/cli/tests/profile_filter_test.cc
+++ b/iamf/cli/tests/profile_filter_test.cc
@@ -11,6 +11,7 @@
  */
 #include "iamf/cli/profile_filter.h"
 
+#include <array>
 #include <cstdint>
 #include <initializer_list>
 #include <list>
@@ -54,7 +55,8 @@ const uint32_t kCommonMixGainParameterRate = kSampleRate;
 const uint8_t kAudioElementReserved = 0;
 const int kOneLayer = 1;
 
-const std::vector<DecodedUleb128> kSubstreamIdsForFourthOrderAmbisonics = {
+constexpr std::array<DecodedUleb128, 1> kZerothOrderAmbisonicsSubstreamId{100};
+constexpr std::array<DecodedUleb128, 25> kFourthOrderAmbisonicsSubstreamIds = {
     0,  1,  2,  3,  4,  5,  6,  7,  8,  9,  10, 11, 12,
     13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24};
 
@@ -369,7 +371,7 @@ void InitializeDescriptorObusForOneMonoAmbisonicsAudioElement(
   AddLpcmCodecConfigWithIdAndSampleRate(kCodecConfigId, kSampleRate,
                                         codec_config_obus);
   AddAmbisonicsMonoAudioElementWithSubstreamIds(
-      kFirstAudioElementId, kCodecConfigId, {kFirstSubstreamId},
+      kFirstAudioElementId, kCodecConfigId, kZerothOrderAmbisonicsSubstreamId,
       codec_config_obus, audio_elements);
   AddMixPresentationObuWithAudioElementIds(
       kFirstMixPresentationId, {kFirstAudioElementId},
@@ -384,8 +386,8 @@ void InitializeDescriptorObusForOneFourthOrderAmbisonicsAudioElement(
   AddLpcmCodecConfigWithIdAndSampleRate(kCodecConfigId, kSampleRate,
                                         codec_config_obus);
   AddAmbisonicsMonoAudioElementWithSubstreamIds(
-      kFirstAudioElementId, kCodecConfigId,
-      kSubstreamIdsForFourthOrderAmbisonics, codec_config_obus, audio_elements);
+      kFirstAudioElementId, kCodecConfigId, kFourthOrderAmbisonicsSubstreamIds,
+      codec_config_obus, audio_elements);
 
   AddMixPresentationObuWithAudioElementIds(
       kFirstMixPresentationId, {kFirstAudioElementId},
diff --git a/iamf/cli/tests/rendering_mix_presentation_finalizer_test.cc b/iamf/cli/tests/rendering_mix_presentation_finalizer_test.cc
index 7675c0f..6efabfc 100644
--- a/iamf/cli/tests/rendering_mix_presentation_finalizer_test.cc
+++ b/iamf/cli/tests/rendering_mix_presentation_finalizer_test.cc
@@ -11,6 +11,7 @@
  */
 #include "iamf/cli/rendering_mix_presentation_finalizer.h"
 
+#include <array>
 #include <cstddef>
 #include <cstdint>
 #include <filesystem>
@@ -175,12 +176,12 @@ constexpr uint8_t kCodecConfigBitDepth = 16;
 void InitPrerequisiteObusForStereoInput(
     absl::flat_hash_map<DecodedUleb128, CodecConfigObu>& codec_configs,
     absl::flat_hash_map<DecodedUleb128, AudioElementWithData>& audio_elements) {
-  const std::vector<DecodedUleb128> kStereoSubstreamIds = {0};
+  constexpr std::array<DecodedUleb128, 1> kStereoSubstreamId{0};
 
   AddLpcmCodecConfigWithIdAndSampleRate(kCodecConfigId, kSampleRate,
                                         codec_configs);
   AddScalableAudioElementWithSubstreamIds(kAudioElementId, kCodecConfigId,
-                                          kStereoSubstreamIds, codec_configs,
+                                          kStereoSubstreamId, codec_configs,
                                           audio_elements);
 
   // Fill in the first layer correctly for stereo input.
@@ -195,12 +196,12 @@ void InitPrerequisiteObusForStereoInput(
 void InitPrerequisiteObusForMonoInput(
     absl::flat_hash_map<DecodedUleb128, CodecConfigObu>& codec_configs,
     absl::flat_hash_map<DecodedUleb128, AudioElementWithData>& audio_elements) {
-  const std::vector<DecodedUleb128> kMonoSubstreamIds = {0};
+  constexpr std::array<DecodedUleb128, 1> kMonoSubstreamId{0};
 
   AddLpcmCodecConfigWithIdAndSampleRate(kCodecConfigId, kSampleRate,
                                         codec_configs);
   AddScalableAudioElementWithSubstreamIds(kAudioElementId, kCodecConfigId,
-                                          kMonoSubstreamIds, codec_configs,
+                                          kMonoSubstreamId, codec_configs,
                                           audio_elements);
 
   // Fill in the first layer correctly for mono input.
diff --git a/iamf/cli/tests/wav_sample_provider_test.cc b/iamf/cli/tests/wav_sample_provider_test.cc
index 65c8679..b94a1c8 100644
--- a/iamf/cli/tests/wav_sample_provider_test.cc
+++ b/iamf/cli/tests/wav_sample_provider_test.cc
@@ -15,7 +15,6 @@
 #include <cstdint>
 #include <filesystem>
 #include <string>
-#include <vector>
 
 // [internal] Placeholder for get runfiles header.
 #include "absl/container/flat_hash_map.h"
@@ -43,6 +42,7 @@ using testing::DoubleEq;
 using testing::Pointwise;
 
 constexpr DecodedUleb128 kAudioElementId = 300;
+constexpr DecodedUleb128 kSubstreamId = 0;
 constexpr DecodedUleb128 kCodecConfigId = 200;
 constexpr uint32_t kSampleRate = 48000;
 
@@ -102,9 +102,8 @@ void InitializeTestData(
   codec_config_obus.clear();
   AddLpcmCodecConfigWithIdAndSampleRate(kCodecConfigId, sample_rate,
                                         codec_config_obus);
-  const std::vector<DecodedUleb128> kSubstreamIds = {0};
   AddScalableAudioElementWithSubstreamIds(kAudioElementId, kCodecConfigId,
-                                          kSubstreamIds, codec_config_obus,
+                                          {kSubstreamId}, codec_config_obus,
                                           audio_elements);
 }