Try wire tap from ambrosia sortware.It is free and will record anything playing through your computer.I used it to make cd's of dvd's I own so I can listen to them in the car and at work. Good luck
First use 0SEx, select the audio that you want then use either a52decX or mAC3dec depending on the type of audio to convert it to AIFF, the nyou can do whatever you wnat with it.
you could rip the whole DVD then just extract the audio if you want. i use MacTheRipper (awesome name or what? ) to rip the DVD then DVD2oneX to compress it. actually... i just checked... sorry, doesn't look like those two apps will do what you want.
i know that there is (should) be an easy way to extract the complete audio tracks from the DVD, try asking in a DVD specific forum if the above mentioned solutions don't suit your needs.
actually, i just DVD2oneX again, it does let you select different audio tracks, but it won't let you start encoding if you don't have any video selected. perhaps that could be a feature request?