I'm thinking the modulation input would come to the Tesla coil as data on a serial line. RS-232 isn't completely dead, you know, and I still have an old Black Box breakout unit for probing and testing voltages on a serial line, as well as an oscilloscope. Now -- how to output audio as a voltage on a serial port? It would have to be uncompressed, analog... oh wait -- what about just using line-out voltage from the sound card? That might do it. No, no good -- can't make it portable. Well, maybe. Still developin'.