I did it on After Effects, indeed i used Expressions but also used a plugin called Trapcode Particular, you can use other particle creation plugin anyway (but Trapcode Particular is the best imho). First you have to put your music into wav file (AE don't like mp3 :P) then you add a audio filter (bass and treble) and put bass at 100 and treble at -50 and convert audio to keyframes, then you put bass at -50 and treble at 100 and do the same; you will have now the music, the bass layer and the treble layer, then you animate with expressions by linking like a word to bass and a picture to treble. In this video i liked two layers with Trapcode Particular, one in Bass and another in Treble and tweaked some Trapcode Particular settings.

Soon on va-mag.net/blog i will post a video tutorial on audio reactive videos on After Effects (sadly i don't know processing :( )

Hope it helped ^^ and thanks for the feedback