If the audio file was downloaded or converted poorly, its header might contain metadata that Avidemux cannot parse.
If you need to add an MP3 to an MP4 without re-encoding, use MKVToolNix to create an MKV file (which happily accepts MP3). Then use Avidemux to edit that MKV. avidemux cannot use that file as audio track
By understanding the relationship between audio codecs, container formats, and Avidemux’s “Copy vs. Encode” behavior, you’ll never be blocked by this error again. Keep a copy of FFmpeg or Audacity handy, and you can convert any audio file to an Avidemux-friendly track in under a minute. If the audio file was downloaded or converted