One of the most common frustrations users face is the dreaded red banner or pop-up stating: You’ve selected an external audio file (like an MP3, WAV, or AAC) to replace or merge with your video’s audio, but Avidemux refuses to cooperate. Why? And more importantly, how do you fix it?
Temporarily change Video Output to Mpeg4 ASP (Xvid) or Mpeg4 AVC (x264) and set Configure to Constant Rate Factor = 22 . Then add the audio. If it works, the original video’s structure was broken. (You will have to re-encode the video, which is slow.) The Duration Mismatch If your external audio file is shorter or longer than the video by a fraction of a second, Avidemux’s copy mode may panic. avidemux+cannot+use+that+file+as+audio+track
This article will dissect every possible cause of this error, from container confusion to codec conflicts, and provide step-by-step solutions to get you back to editing. Before fixing the error, you must understand how Avidemux thinks. Unlike modern editors that use a timeline and re-encode everything seamlessly, Avidemux is a smart copy tool. It works best when it doesn't have to convert data. One of the most common frustrations users face
If yes, the problem was a codec mismatch. You can now change Audio Output from PCM to Copy or MP3 and test again to find the optimal setting. Temporarily change Video Output to Mpeg4 ASP (Xvid)