How to build disco lights?
Your question stirred some cobwebs deep in the recesses of my dusty brain. I seem to remember building such a thing myself way back when. I don't remember the specifics, but if I recall correctly, we used RC filters to segment the incoming audio signal and SCRs to trigger short strings of 110V AC Christmas lights. Probably a transistor to drive the SCR. These things were all the rage, along with strobe lights, back in the late 70's.