Its not just Youtube you can get mp3 s of video from Myspace, Vimeo, Metacafe and many more.

You just need to visit vidtomp3.com

Just type the url of the youtube video [or any other video online] and it will take some time and you will be able to download its mp3 file.

Eg I want to download the sound track of this youtube video

http://www.youtube.com/watch?v=IsvzYbhmo2g

i just need to copy the url above and paste it and hit the download button.

It will take sometime making ready your mp3 file for download

And then the download mp3 button will appear, click and your mp3 file will be downloaded.

Visit now vidtomp3.com to convert video files to mp3