| Output Container | Supported Audio Codecs (for direct copy) | | :--- | :--- | | | AAC, MP3 (limited), AC-3, PCM | | MKV (.mkv) | Almost everything (MP3, AAC, FLAC, Opus, DTS) | | AVI (.avi) | MP3, PCM, AC-3 (AAC is very problematic in AVI) |
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
Go to Format dropdown in the main toolbar and select MKV V2 or MKV . Then try adding your external audio track again. Avidemux’s "Copy" mode hates Variable Bitrate (VBR) MP3 files. When you cut video on keyframes, VBR audio loses sync because the bitrate fluctuates. Avidemux is old-school; it prefers Constant Bitrate (CBR). | Output Container | Supported Audio Codecs (for
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. (You will have to re-encode the video, which is slow
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.