Oh my God! This is the stuff nerd's dreams (at least mine...) are made out of: two singing tesla-coils (a.k.a. "zeusaphones") playing the super mario bros. theme. It is something i could never have imagined being build/performed but always knew i wanted to happen my whole life. As for how it works, this is the description from one of the attendees of the Duckon 2007 Teslathon, where Steve Ward first presented his (single) singing coil :

"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.

What's not immediately obvious in this video is how loud this is. Many people were covering their ears, dogs were barking. In the sections where the crowd is cheering and the coils is starting and stopping, you can hear the crowd is drowned out by the coil when it's firing.

This Tesla coil was built and is owned by Steve Ward. Steve is a EE student at U of I Urbana-Champaign. He and Jeff have been going to Teslathons, which is where they met..."

(quotation taken from hauntedfrog.com)

another description taken from the Makezine Blog:

"The music that you hear is coming from the sparks that these two identical high power solid state Tesla coils are generating.

There are no speakers involved. The Tesla coils stand 7 feet tall and are each capable of putting out over 12 foot of spark.

They are spaced about 18 feet apart. The coils are controlled over a fiber optic link by a single laptop computer. Each coil is

assigned to a midi channel which it responds to by playing notes that are programed into the computer software."

Damn... might be a good idea to use for my album.. >)