First step is to get the video off youtube, and onto your PC. This can easily be done here: http://www.ripzor.com/youtuberipper.html
Just follow the steps there. Once you have the file (and renamed it to *.flv) you'll need to rip the audio from it, the best application I've found for this is this: http://www.moitah.net/download/latest/FLV_Extract.zip
It will automatically export the audio as an mp3.
or you can use program to record all sounds on pc lol. That's what I'm gonna do, but SAB's method is going to be better quality and probably in the end, easier.