AbecX's suggestion will work fine if you don't have a DVD rom on the computer.
But if you want a perfect digital copy, you'll need a DVD rom on your computer, and get a program called Gordian Knot. That will take you through the steps of ripping and encoding the DVD. If you want just the audio, you'll still have to rip the DVD, but when you get to the encoding phase, just add the audio to the job queue, and it will only encode the audio.
Then you're good to go.
Actually, the program(s) you'd need are BeSweet to encode the audio, SmartRip or DVD Decrypter for ripping the dvd, and DVD2AVI to extract the AC3 audio files from the vobs you just ripped from the DVD ... but they all come with Gordian Knot anyway, so if you ever want to back up DVD's at a later time, you've already got all the tools you'll need.
Graphic Design / Photography / Web Design