I'm working of amv decoder (and in later phase encoder) which should be scriptable and doesn't require human interaction. Oh, it should also work on Linux, so it's written in perl.
I would appreciate help with audio format: it seems that it's some variant of ADPCM (and not mp3 frames!), but I haven't figured out which one (there is 8 bytes of something in front of each ADPCM frame).
Idea is write filter which will be able to convert amv movies to avi on-the-fly (without recoding) and convert correctly formatted avi file (mjpeg frames, correct audio encoding (if it's standard) and framerate) to .amv without recoding.
One of reasons for requesting correct input size and framerate is that we have to pre-process movies anyway, and ffmpeg and mencoder does that task so much better than windows AVM transform