If you use the Firefox browser, which you should be able to get or upgrade here:
http://www.mozilla.com/en-US/firefox/upgrade.htmlyou can get the Download Helper plugin, which lets you copy YouTube videos, images, and just about any media onto your hard drive:
http://www.downloadhelper.net/You should be able to download the high quality version of any of those videos, if it's available, and it should have better sound/visual.
I imagine there are similar plugins for Internet Explorer, Safari, Opera, and other browsers, but I don't have experience with them.
Once you have the video on your hard drive, you can use GarageBand or iMovie (if you're a Mac user) to separate out the audio component and save it as a separate file. I don't know what Windows tools are out there.
Jeff