AI: WebRTC: Support G.711 (PCMU/PCMA) audio codec for WebRTC. v7.0.124 (#4075)

[ossrs-ai]

This PR adds G.711 (PCMU/PCMA) audio codec support for WebRTC in SRS, enabling relay-only streaming of G.711 audio between WebRTC clients via WHIP/WHEP. G.711 is a widely-used, royalty-free audio codec with excellent compatibility across VoIP systems, IP cameras, and legacy telephony equipment.

Fixes #4075

Many IP cameras, VoIP systems, and IoT devices use G.711 (PCMU/PCMA) as their default audio codec. Previously, SRS only supported Opus for WebRTC audio, requiring transcoding or rejecting G.711 streams entirely. This PR enables direct relay of G.711 audio streams in WebRTC, similar to how VP9/AV1 video codecs are supported.

Enhanced WHIP/WHEP players with URL-based codec selection:

# Audio codec only
http://localhost:8080/players/whip.html?acodec=pcmu
http://localhost:8080/players/whip.html?acodec=pcma

# Video + audio codecs
http://localhost:8080/players/whip.html?vcodec=vp9&acodec=pcmu
http://localhost:8080/players/whep.html?vcodec=h264&acodec=pcma

# Backward compatible (codec = vcodec)
http://localhost:8080/players/whip.html?codec=vp9

Testing

# Build and run unit tests
cd trunk
make utest -j && ./objs/srs_utest

# Test with WHIP player
# 1. Start SRS server
./objs/srs -c conf/rtc.conf

# 2. Open WHIP publisher with PCMU audio
http://localhost:8080/players/whip.html?acodec=pcmu

# 3. Open WHEP player to receive stream
http://localhost:8080/players/whep.html

Related Issues

• Fixes [Feature] WebRTC: Support G711A audio codec #4075 - WebRTC G.711A Audio Codec Support
• Related to [Feature] WebRTC Support VP9 #4548 - VP9 codec support (similar relay-only pattern)