Converting your file to an uncompressed, standard WAV format bypasses all demuxing restrictions in Avidemux. Cannot use mp3 as audio track - Avidemux.org
ffmpeg -i problematic.mp3 -c copy fixed.mp3 avidemux cannot use that file as audio track
This article will explain exactly what's going on behind the scenes, list the specific audio file types Avidemux can and cannot use, and provide a step-by-step guide to getting your project back on track. Converting your file to an uncompressed, standard WAV