It would be nice if Avidemux could save in MKA format. If the user selects MKV output format and disables the video track keeping only the audio track, Avidemix should save the file in MKA format.
There is no way to disable video track in Avidemux, IMHO it is not meant to be a universal swiss knife like command-line FFmpeg. "A" in MKA is not a different format from MKV, it is just a hint that the container lacks a video track.
If in "Output Format" there was the option to save in MKA, Avidemux itself should disable the video track automatically.