couple of ways, but might require more than one step...
There are sites that can turn a YT vid into an MP3 file...
http://flvto.com/ is the best one I've found... Then you can use something called Amazing Slowdowner to change pitch/speed. Great app.
http://www.ronimusic.com/ That's what I do, anyway.
Others have used Audacity to do both processes.
http://audacity.sourceforge.net/ I've used Audacity to break up big MP3's into smaller files (to separate individual tracks from a jam or rehearsal or performance), as well as to import music from analog sources (tape or LP), but I've never used it for YouTube conversion or slowing down music.
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