See also http://everynoise.com/ which is a view into how Spotify classifies music.

The creator wrote about it here:

http://blog.echonest.com/post/52385283599/how-we-understand-...

and writes a lot about it on their blog:

http://www.furia.com/page.cgi?terms=noise&type=search

Of course those are going in the other direction, not generating the classification from the data, but it's probably one of the best data sets as far as classifying existing music.