TubeTV is a free app that will download embedded videos (mainly from YT), and convert them to a size/format of your choice (with the help of Perian, free also) while adding it to iTunes as well. I have mine set to convert everything for my iPhone.
From there, if you have the newest version of GarageBand (may work on the previous versions, I'm not sure), you can add videos to it, which GB will automatically separate out the audio. From there it's just deleting the video track, and then sharing/converting it to iTunes.
Took me 20 minutes to figure it out, but is a 5 minute process at most depending on the length/size of the video (and your Mac).