Inspired by awesome Oskar Fischinger works, I decided to make my own experiment in After Effects using Isaac Albeniz's music.
More about Fischinger: http://www.oskarfischinger.org/
More about Albéniz: http://en.wikipedia.org/wiki/Isaac_Alb%C3%A9niz
I'm not sure who is playing on the piano, probably it's Alicia de Larrocha, an excellent pianist, if not the best player of Granados and Albéniz (mainly, but lots of other authors) of her time.
More about Alicia de Larrocha: http://en.wikipedia.org/wiki/Alicia_de_larrocha
Animation process started by filtering the song in 3 main frequencies. A low, high and band-pass filters were used. Then audio was converted to keyframes. Each box is animated mainly by expressions.
The rotating boxes were animated by expressions as well. Instructions to do so were taken from AE Tips and Tricks (http://hottek.net/2007/11/square-wheels.html)
(Many thanks to the author of the blog).
Background image is from Fudgegraphics. I took it from its Deviantart Page. (http://fudgegraphics.deviantart.com/)