Piece of Cake!
Replay Media Catcher can do this ALL for you - it can quickly download youtube videos, then once they're recorded - you can either extract the mp3 audio from the file, or simply convert the video to an audio or video file of your choice.
I know, it sounds too easy to be true - but it really IS that easy. Check out the free demo version here, you'll see what i mean:
Replay Media CatcherGood luck!
Media Guru