Fix: Convert your audio to first (Audacity or ffmpeg). Avidemux hates MP3/M4A as external tracks.
When you add an external audio track, Avidemux does not want to transcode it on the fly. It wants to the audio stream directly into the output container (MKV, MP4, AVI). For this to work, three conditions must be met: avidemux+cannot+use+that+file+as+audio+track
: Older versions of Avidemux (e.g., 2.7.x) had known bugs related to external audio track recognition. : Upgrade to the latest stable release (e.g., Fix: Convert your audio to first (Audacity or ffmpeg)