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.
Happy editing.
Avidemux is a beloved tool in the video editing world for one simple reason: it’s fast, free, and perfect for cutting, filtering, and re-encoding video without the bloat of professional suites like Premiere Pro or DaVinci Resolve. However, its simplicity has a downside. When something goes wrong, the error messages can be cryptic, unhelpful, and infuriating.
Convert your VBR MP3 to CBR MP3 or PCM WAV using a tool like Audacity or FFmpeg before loading it into Avidemux.
If you are trying to use a FLAC or Opus file as an external track, you must change your output format to MKV . MP4 does not support FLAC natively.
FFmpeg one-liner: ffmpeg -i input_vbr.mp3 -b:a 192k -acodec libmp3lame output_cbr.mp3 Avidemux uses a lightweight parser. It doesn't handle malformed ID3 tags, unusual sample rates (e.g., 44.1kHz is fine, but 8kHz might fail), or files that start with junk data.
