Click the drop-down menu for Track 1 and choose Add Audio Track .
Check , then check Play another media synchronously . Click Extra media and add your external audio file. avidemux+cannot+use+that+file+as+audio+track
If your main goal is simply to combine a video and an audio track without any further editing, MKVToolNix is a fantastic alternative that bypasses Avidemux's limitations entirely. Click the drop-down menu for Track 1 and