First get DVD Decrypter (it's on
www.doom9.org's software page). Make sure it's in mode>IFO I and then find the right VTS and chapter that the song is in (a good way is to find it in powerdvd, the title = vts and chapter = chapter).
Once you have the right chapter selected hit Stream Processing and check Enable Stream Processing. Deselect all the streams except the audio that you want and select Demux for that stream.
Rip it and you have the ac3. You can convert it to wav in Besweet (
http://dspguru.doom9.org )