For anyone interested,
I also just recently figured out how to do this. I'm real excited that I can now get audio from things like Bobby McFerrin's song w. the muppets in the Jim Henson Hour, and the like.
There are a number of different ways, but the method I use is from here:
http://www.zeropaid.com/news/7469/How+to+Download+Videos+From+YouTube+and+MySpace
This link tells you to get the Mozilla Firefox Internet Browser (which is free and works well). It also recommends the VLC player (also free) which is very handy. After you have that, follow the instructions, and then you can get what you need! Then, it's a matter of formatting (converting for just the sound, converting the video, etc.) For this I use something called SUPER (Also free and not too hard to use once you get the hang of it - takes a little practice). The biggest pain is downloading SUPER as it takes you through a few hoops, for example you have to scroll down to close to the bottom of the page to find the download link. But trust me, it's really worth it - it's a great tool. You can download it here:
http://www.erightsoft.com/SUPER.html
Hope that is helpful to some of you!