I've done this for my film. I used a sound program that came free with my sound card.
Creative Wave Studio
I played the DVD on my computer and then hit the record button on my sound recording program that was set to record, not from the mic, but from "what U hear" option.
You can always use windows sound recorder. It has limitations, but it works.
If you don't have a DVD player in your computer, then I would recommend that you get one.