Instead of adding the file as an external track immediately, try this: Open your video file. Go to . Check the box for the track you want to replace. In the dropdown, select Add audio track and pick your file.
When you add an external audio track, Avidemux doesn't look for a "song file" (like an MP3). Instead, it looks for a that it can directly copy into your video container. This means the audio file must be in a very specific, uncompressed, or raw format. It essentially acts as a digital welder, not an audio player, joining pre-existing parts. avidemux cannot use that file as audio track
Ensure that the audio file exists and that the path to the file is correct: Instead of adding the file as an external