I just read this at PC World:

Rip Music Off a DVD

Is there a way to rip audio from a DVD onto a PC? I record songs to tape, and from there to a PC.

Tommy Kikuchi, Woodside, New York

Unfortunately, the Digital Millennium Copyright Act calls into question the legality of convenient DVD rippers such as Handbrake. But I know of a better solution than recording to tape first (and it's legal), although because of a DVD's copy protection, it still entails some loss of sound quality. Simply connect one end of a stereo audio cable to your PC's Line Out jack (where you plug in the speakers) and the other end to the PC's Line In jack (where you attach the tape player or turntable). This requires a stereo audio cable with one-eighth-inch miniplugs on both ends. If you don't already have one, you can buy such a cable for less than $5 at any electronics store.

Once you're connected, rip the music with your choice of recording software. The PC will be silent until you plug the speakers back in, but your recording program's VU meter will tell you that there's sound. Remember that selling or giving away your digital recordings of any copyrighted material is illegal.
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A cable from Line Out to Line In and record. I never would have thought of that. Anybody ever try this?

If you are using a normal DVD to rip audio, you can use MPlayer to losslessly dump the DVD audio onto a AC3 file.

Example:

mplayer dvd//2 -dumpaudio -dumpfile Test.ac3

no i never did but i'm sure it's lossy...
i can't answer the "legal aspect" but you're getting the soundtrack in the end....
why not dvddecrypter splitting by chapter(where each chapter is one music) and in stream processing select the desired audio format? ...(or all audios)




edit: i found one guide... http://www.doom9.org/index.html?/dvddec.htm