Edit: The above method is smarter.
http://www.jakeludington.com/ask_jake/20050216_export_dvd_audio_to_cd.html
Original post if anyones interested:
It's possible but can be tricky because there are several ways music can be stored on a DVD, not just one as with CD:s, so there are many ways to rip the music.
A DVD is made of chapters and tracks. The first thing you should do is to play the DVD on your computer. Most DVD-playing software can switch between chapters (and tracks) in menus somewhere. What you need to do is locate the correct track(s) in the correct chapter. If you can't find it, then the audio is probably "dvd audio" and if it is, disregard everything in this post.
Once you have located the correct tracks and chapter you need the excellent tool DVDDecrypter. You can download it from here:
http://www.afterdawn.com/software/video_software/dvd_rippers/dvd_decrypter.cfm Unfortunately the program is a bit tricky to work with. Look at the first part of this guide to get familiar with the interface:
http://club.cdfreaks.com/showthread.php?t=139536
On the right (not left) part of the program you can chose between chapters and tracks. Select the correct one and rip it as shown in the guide.
It gets fuzzier from here, depending on what format the audio track is. Try to play the ripped audio track in your DVD player software. If it works, then the first step is done. Now the file needs to be converted somehow. This depends slightly depending on the format and file extension.