Here is a reliable workflow to bypass the "Cannot use that file as audio track" error and get your audio muxed correctly.
High-end audio tools (like Traktor) add metadata headers that can confuse Avidemux. Re-saving the file without metadata often fixes this. 💡 How to Add the Track Correctly Once your file is in a compatible format (WAV or MP3): How to add audio to video with VLC and Avidemux avidemux cannot use that file as audio track
A user on the Avidemux forum encountered the error with their AAC track, and it was confirmed to be because they were trying to use "an audio-only MP4 container with an aac stream inside," a format that is simply not supported. Here is a reliable workflow to bypass the