Is there a utility that will extract the audio from a MPG or FLV?

Is there a utility that will extract the audio from a MPG or FLV (such as something I Nabbed from YouTube) and save it as a MP3 or WAV? There’s a lot of music I discover that I like attached to bizarre You Tube videos that I would like to add to my MP3 collection. If the song title is given in the video, I try to buy the MP3 from Amazon for $0.99 if it’s there, but the song isn’t always named. I have no problems capturing the videos and saving them on my computer, so that’s not the issue. I just want to extract the audio from an existing MPG or FLV and save it as MP3…