IT ...

1. Encode sound to make sure it's mp3, skip video to get a .avi with sound only.
mencoder -oac mp3lame -ovc frameno -o aa1.avi vts1.vob

2. Dump the raw sound out from .avi
mplayer -ao pcm mplayer -ao pcm -vo null -vc dummy -dumpaudio -dumpfile aa1.mp3 aa1.avi