MJPG - Muxer Cannot Open

[turtle-purtle]

Windows 7; Avidemux 2.7.5 - first time install.
I've not used this before, used to use virtualdub.

I have a file encoded with MJPG, 640*480, 1:1 aspect ratio, 20fps.
Audio: 8bit PCM Mono, 11024Bps / 88kbps

The video plays fine in Avid, but when I try and play it, even just with Copy/Copy for video/audio, I get an error: Muxer - Cannot Open. Pressing Ok brings up another error dialog - "Failed - X.mkv was NOT saved correctly".
Thoughts? The search only finds one other result for MJPG.

[eumagga0x2a]

The problem is most likely the audio track. Please test whether it works when you encode it e. g. to mp3.