I've used DVD Decrypter (I'm not going to provide a link, you'll have to look for it yourself) to accomplish that in the past. Use IFO mode, find the clip you want in the Input tab, and use the stream processing tab to isolate the audio (select only the relevant audio tracks, and use the demux option.) (I don't have access to the program right now, so I might be missing a step or two...)
If everything goes smoothly, you should be left with either a plain wave file or a Dolby Digital AC3 file, depending on the audio format used on the DVD. If it's a wave file, you're pretty well good to go. If it's an AC3 file, use something like AC3Filter or HeadAC3he to convert it into a wave or MP3.