Just an idea, I don't know if it'll work, but get yourself a double-ended audio jack and plug one end in your speaker output and the other in your mic imput. Find a good MP3 recorder (I don't know of any, any help out there?) and set it to record, then start your movie. Hypothetically, your comp will loop on itself and save the sound from the movie on there. Downside: You won't hear anything while it's recording.
But once that's done, I think you'll have a soundtrack. I'll try it on my comp sometime and get back to you.