The best method, using only free softwares, I can think of is:
1> First rip the video (or part of it) from the disk to AVI.
2> Then seperate audio from video using Virtualdub
3> Crop the audio in audacity (install lame.dll plugin first) and export to mp3.
All this is a lot of work, so best of luck!
Some guides wich may help with the above steps:
*www.wikihow.com/Rip-a-DVD
*www.doom9.org/index.html?/mpg/ifoedit-menustripping.htm
*www.doom9.org/index.html?/audio-guides.htm
*www.doom9.org/virtualdub_procedures.htm
*audacity.sourceforge.net/manual-1.2/tutorials.html
There may be easier methods out there but I am not aware of them.