Do you mean nick a bit of the movie's soundtrack, or do you mean separate the voice from the background noise or music in the movie?
To essentially sample a DVD, you can use any recorder (even windows Voice recorder should be able to do it), Wavelab, Cool Edit, Soundforge, or sequencer (Logic, Cubase), so long as the record input is set to your soundcard's monitor (i.e. recording anything that you can hear, rather than an audio input). If you don't have the software there are definitely free wave editors around on the net that'll do it.
If you want to actually separate the sound, this is much more difficult. A decent equalizer/filter is what you want, though there's no guarantee that you can literally cut the voice out, as it depends on the frequencies of voice and any background noise or music that you want to keep.
Hope this helps.
