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? 😉