Here is a quick tip for extracting audio from youtube videos. I assume you have a reasonably recent linux distribution. (Things might work on other operating systems as well as all the applications that I mention below are ported to other operating systems)
Step 1: Download the flash video file from youtube. Either use greasemonkey script or a python script to extract it. (I use the python script way BTW) You can also use the online site VideoDL.org to show you the download link.
Step 2: Use ffmpeg or mplayer to extract audio from the flv file that you just downloaded:
ffmpeg -i f7v9NhmF3SY.flv johny.mp3
mplayer -dumpaudio f7v9NhmF3SY.flv -dumpfile johny.mp3