Avidemux Cannot Use That File As Audio Track -

By setting Audio Output to “AAC” instead of “Copy,” Avidemux stops trying to preserve the original audio format and instead creates a brand new, container-friendly audio stream.

If an MP3 still fails, the file might have a large header. Community experts suggest that stripping the first few kilobytes of the file (which often contain non-standard metadata) can sometimes allow Avidemux to "see" the actual audio frames. Supported External Audio Tracks avidemux cannot use that file as audio track