There are several software titles out there to accomplish this. Just do a google search for DVD Audio Rippers and you will find one suitable for what you need. I haven't seen any that are shareware/freeware as of yet.
DVD Audio Extractor( rip to OGG, MP3 or Wave file or demux audio streams directly to LPCM wav, mpg, ac3 or dts files) DVD TO AC3 RIPPER (convert DVD audio to AC3) Alternativly i think you can just use dvd decrypter demux process and rip the audio to AC3(not tested)