4 short, differently-pitched beeps are each given a unique, slightly-off rate -- anywhere between 5000 and 5050 milliseconds -- to clock. Over time they go out of phase, and come back in phase at some point. Each sound is designated to one speaker, for a total of 4 speakers tiered on 4 different levels in roughly a vertical line.
Video taken from show with people around.
(thanks to PB for the video + derek f. for helping me strike)