Need to make a couple of short audio files from video clips.

(EDIT: I was told to use AUDACITY.) Does anyone have a good tool for capturing audio, such as from a video playing on the same computer? Don't care if it runs on Windows, OS/X, or Linux, I have plenty of computers to use it on and I only need to use it once. (Maybe twice, if I can find a DVD copy of Ghostbusters.) ... Now I'm wondering if I can just have one thing running the computer's speakers and another thing listening in on the mic? Wouldn't be the greatest audio quality but the ultimate destiny for this project is just a ringtone or two.