Twin Solid State Musical Tesla coils playing Mario Bros theme song at the 2007 Lightning on the Lawn Teslathon sponsored by DC Cox (Resonance Research Corp) in Baraboo WI. 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.
Basically the signal is sent through a filter which turns a switch on or off every time the wave crosses zero ( think of a sine wave). So a 44 KHz signal would cause the tesla coil to switch on and off 44 thousand times a second. I saw a video of this a while back, and that seemed to be the gist of how it worked.