This is a fairly simple conversion, and can be done in several different ways, which makes it all the more surprising that the questions comes up fairly often in VO forums.
This particular step-by-step process on a WIRED site is as good as any.
BTW, this formula expects you to have video converter software installed. If you don’t already, I’d highly recommend any of the AVS products. They have great audio, video, and photo utility software for an incredible price (IMHO).
CourVO
See this Amp at http://bit.ly/ftJ1U8
http://howto.wired.com/wiki/Download_and_Convert_YouTube_Video…
(ed note: 12/26/11; another source for this contributed by “Luke”: http://www.youtube-to-mp3.org/)