Well, you get into the same problems as with ripping video off DVD. In fact, it's the exact same thing, as the audio is encoded multiplexed in the VOB along with the video. So you'd have to get a DVD ripper and then something that could extract the audio from it. My suggestion would be SmartRipper and DVD2AVI. BTW, it isn't obvious in DVD2AVI as to how to save a WAV. You need to ``Save Project'' after selecting the right things. Also note the 48kHz->44.1kHz option.

Keep in mind that this is possibly illegal even if you're just doing it for backup or compatibility purposes, as you say. Gotta love the US legal system and it's international cronies.