There's a little info contained in the video description:
"This is a solid-state Tesla coil. The primary runs at its resonant frequency in the 41 KHz range, and is modulated from the control unit in order to generate the tones you hear.
"So just to explain a little further, yes, it is the actual high voltage sparks that are making the noise. Every cycle of the music is a burst of sparks at 41 KHz, triggered by digital circuitry at the end of a "long" piece of fiber optics."
Sounds like pulse rate modulation of the 41Khz bursts. Pretty interesting stuff :-) There are more of these, too ..
http://www.youtube.com/watch?v=VX5V_9s0Gfw&NR=1 and this one has an actual description of a similar coil
http://youtube.com/watch?v=cOkTrqTd0hw&mode=related&search=
