There are loads of web-based services for creating an MP3 file from a Youtube video, such as these:
http://www.video2mp3.net/
http://www.listentoyoutube.com/
There are also programs, which you can put on your PC, which will do the job. e.g.:
http://download.cnet....5553.html?tag=mncol;2
The easiest way though is to use Firefox as your browser, with the Easy Youtube Video Downloader 4.1 add-on. (It just needs a couple of clicks to save any Youtube video to your PC, whether as an MP4 video file, an FLV one, or an MP3 audio file).
Chris