what I would do is if you have Adobe Audition (formerly Cool Edit Pro) you can pop the DVD in and open it through Adobe. It should create an audio file and what looks like an attempt at the actual video file, but Adobe doesnt do that so it comes out weird. You can ignore that file and just save the audio file as a wav or mp3 or whatever you like. If you purchase the DVD then I would think that this is a legit move. Forgive my kneiving mind if it's not. I figure if you bought the DVD you have rights to use it in any way you please.
Heck, if you're a DVD Mixer or using pro DVD players with a switcher, you can just play the DVD and put ambient or crowd cam over the screen since it's such a naughty flick. just a suggestion.