In reply to keith-ratcliffe:
There are various online services that do the conversion for you, like
http://www.youtube-mp3.org/.
Do a search and you'll find there's quite a few of them. I imagine they'll be more up to date than software.
There are also various browser plugins like video download helper (not used for a while but you might need a paid version to split of the audio automatically). Or you could use something else to extract the audio.
You might also check out Soundcloud.com, which is basically Youtube but for audio files rather than video files. I'm always surprised that people add stuff to Youtube when there's no actual video component.