I recently stumbled across this video on vimeo by Mattias Peresini, in which he has mashed together Trapcode Particular, Trapcode Sound Keys, and After Effects to create this program that responds to music.
The result… stunning visualizations. I only wish the video lasted longer.
According to his Vimeo page the project will be available soon to download.
The song for those who are curious is: Erlend – Ted & Francis