Instead of using Audio > Select Track , try a different workflow to merge audio:
: Use a tool to strip metadata or "clean" the audio file by re-saving it in a basic audio editor. Unsupported Bit Depth or Codecs avidemux+cannot+use+that+file+as+audio+track
: Avidemux cannot use audio files already "wrapped" in a container like .m4a or .mp4 as an external track. It lacks the internal demuxers to strip these for external loading. Instead of using Audio > Select Track ,
: Avidemux cannot use audio that is already inside a container (like : You must provide the "raw" audio stream (e.g., ) or convert the file to a standard before importing. Metadata Interference Instead of using Audio >
New user getting "Cannot use that file as audio track" - avidemux.org