create a DVD audio file (MPEG2) from an AVI file

For this you need mplayer and the mjpegtools. It's a simple two step process:

• extract a WAV file from the AVI:
  mplayer -ao pcm:fast -vo null -vc null <inputfile.avi>
• the audio will have been written to a file audiodump.wav, which you can first edit with a program like audacity if you like
• to convert the WAV file to MPEG2 (the format a DVD uses for audio):
  mp2enc -r 48000 -o audio.mpa < audiodump.wav
• the MPEG2 audio will end up in a file audio.mpa, which is ready for multiplexing with (adding to) an MPEG2 video file (e.g. "film.m2v") using mplex (which is also part of the mjpegtools package)