Condensing audio doesn't work in flatpak on linux

[MinmoTech]

Original report: https://discord.com/channels/752293144917180496/944336185029971978/1125448701331132446

Error
An error occurred. Please start Anki while holding down the shift key, which will temporarily disable the add-ons you have installed.
If the issue only occurs when add-ons are enabled, please use the Tools > Add-ons menu item to disable some add-ons and restart Anki, repeating until you discover the add-on that is causing the problem.
When you've discovered the add-on that is causing the problem, please report the issue to the add-on author.
Debug info:
Anki 2.1.65 (aa9a734f) Python 3.9.15 Qt 6.5.0 PyQt 6.5.0
Platform: Linux-6.3.9-zen1-1-zen-x86_64-with-glibc2.35
Flags: frz=True ao=True sv=2
Add-ons, last update check: 2023-07-04 01:09:58
Add-ons possibly involved: ⁨Migaku Anki Add-on⁩

ERROR:tornado.application:Uncaught exception POST /condense (127.0.0.1)
HTTPServerRequest(protocol='http', host='127.0.0.1:44432', method='POST', uri='/condense', version='HTTP/1.1', remote_ip='127.0.0.1')
Traceback (most recent call last):
  File "/home/julius/.var/app/net.ankiweb.Anki/data/Anki2/addons21/1846879528/lib/shared/tornado/web.py", line 1702, in _execute
    result = method(*self.path_args, **self.path_kwargs)
  File "/home/julius/.var/app/net.ankiweb.Anki/data/Anki2/addons21/1846879528/migaku_connection/audio_condenser.py", line 69, in post
    self.condense_audio(filename, timestamp)
  File "/home/julius/.var/app/net.ankiweb.Anki/data/Anki2/addons21/1846879528/migaku_connection/audio_condenser.py", line 36, in condense_audio
    self.connection.ffmpeg.call(
  File "/home/julius/.var/app/net.ankiweb.Anki/data/Anki2/addons21/1846879528/migaku_connection/program_manager.py", line 61, in call
    return subp.call([self.program_path, *args], **kwargs)
  File "/home/julius/.var/app/net.ankiweb.Anki/data/Anki2/addons21/1846879528/migaku_connection/program_manager.py", line 29, in call
    return subprocess.check_call(*args, **kwargs)
  File "subprocess", line 373, in check_call
subprocess.CalledProcessError: Command '['ffmpeg', '-y', '-f', 'concat', '-safe', '0', '-i', '/home/julius/.var/app/net.ankiweb.Anki/data/Anki2/addons21/1846879528/tmp/1688400657765/list.txt', '-write_xing', '0', '/run/user/1000/doc/7e56e88b/julius/ep03_(1920x1080_Blu-ray_H264_10bit)_[260CDCEB]-condensed.mp3']' returned non-zero exit status 1.
ERROR:tornado.access:500 POST /condense (127.0.0.1) 22.37ms