My hack for the Music Hack Day 2011 in London, a 24h Hackathon for all things music.
SiriProxy is used to intercept the communication with Apple's servers. Notorious Siri then sends Notorious B.I.G.'s Hypnotize to the device.
Siri's speech synthesis is synced to the beat using the timestamps obtained from the Echonest API which were then manually tweaked, to smooth out delays in the text-to-speech engine.
Thanks to Universal Music for awarding me a nice pair of Dr. Dre headphones