If you are talking about ripping audio from a DVD-A disc, forget about it. It can't be done. You could rip the DVD-V portion of the disc if it has one.
If you are talking about just ripping the audio from a regular DVD-V disc, my preferred method is to rip the disc to the hard drive using SmartRipper or DVD Decrypter and DeCSSing it in the process. Then use BeSweet, and the BeSweet front end to grab the audio. I prefer taking the LPCM 2 channel stereo track, since this is lossless. The 5.1 mix will probably be a 384kbps ac3 stream which is lossy.
A great resource is www.doom9.org They should also have links to the necessary software.