Via Joseph Schmitt:

This is the best thing I’ve seen all week. Keep up the good work, Internet:

SiriProxy is used to intercept the communication with Apple’s servers. Based on your song request, Notorious Siri then sends your choice of Notorious B.I.G.’s Hypnotize and an a-cappella rendition of Queen’s Bohemian Rhapsody to the device (the latter requiring 4 iPhones 4S).

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