You'll need a decrypter that can select just the audio stream. DVDDecrypter in IFO mode can do this, however, if the movie has an advanced protection scheme that has come out since development on DVDDecrypter stopped, you'll need to run AnyDVD in the background to break the encryption.
Insert the dvd into your dvd drive. Start DVDDecrypter. Set it to IFO mode. Click on the Stream Processing tab and put a check mark in the little box for Enable Stream Processing. You should see a list of video, audio and subtitle streams in the movie, with check marks in the boxes next to them. Remove the check marks on everything except the audio stream you want to keep. It is probably going to be an AC3 stream. Now start the ripping process by clicking on the disk to hard drive icon at the bottom left.
The result you will get from this will be a Video_TS folder containing an IFO file and a Vob file. The vob file will be your audio stream, and you can safely rename it with an ac3 extension (if it was ac3 originally). Once renamed with the ac3 extension, you should be able to work with it in any audio program that recognizes ac3 files.
Now, you don't say exactly what format you want on the end result, just that you want it on a cd. Do you want an audio-cd that will play on any cd player, or are you just saving it as data to play on the computer? Saving as data is easy...just burn it to the cd.
If you want to go to audio-cd format, you'll need something that can convert ac3....but it may wind up being too large to play as a cd.