One of my previous side projects is the Plasma Speaker. I did it as a final project for my Electrical Circuits 2 class, along with Pedro Borges Costa (also a member of the Robotics and Automation Laboratory).
The idea is generate sound using plasma, basically heating and cooling the air to generate the mechanical waves (sound). I used a TL494 as the modulator, and a monitor Flyback transformer and some MOSFETs to make the plasma arc.
The schematics are available for both the EQ and the driver and modulator.
An early demonstration is also available below: