Even if your codec matches, the container (the file wrapper, e.g., .mp4 , .mkv , .avi ) might reject it.
(e.g., AAC from some containers, Vorbis, Opus, MP2) avidemux+cannot+use+that+file+as+audio+track
Fix: Convert your audio to first (Audacity or ffmpeg). Avidemux hates MP3/M4A as external tracks. Even if your codec matches, the container (the