In Garageband (09 at least), you can add a movie which will then be separated into an audio track and a video track. You can then just delete the video track and use the Share menu to send it to iTunes and it will convert it along the way to aac by default, but it can be set to mp3 as well. The movie has to be a format Garageband will read, of course, which might not include wmv or avi's.
I did the exact same thing you want to do with TubeTV (with Perian) to download and convert a YouTube video into a QuickTime format (specifically for the iPhone), and then added the movie to garageband and just deleted the video track as described above.