<physics_numpty>
What is it about it that makes the tone? I've read the bit where it says the Tesla is connected via Midi to a keyboard - but what is it about the spark that can be varied to play a given tone?
</physics_numpty>
I'd guess that they're modulating the current in the coils with the audio signal - an ionised path like that makes a passable loudspeaker
(I know this from encountering arcing on BBC World Service site*. The voice modulation could be heard clearly)
*that you pass on the Elenith