Maybe I'm thinking about this wrong, but you should be able to play the AVI/MOV file, and use any of the shareware audio programs that have audio recording capabilty to record the audio portion of the file as the video file plays.
This would be the cheapest way to do it. The quality shouldn't be too bad, although it's not going to be that great either. You can try it with the Windows Sound Recorder to hear what it sounds like.