What about electromagnetic/electrostatic valves? The problem is that traditional camshaft-driven valvetrains can only rev so fast before the valve can't keep up, but if you simply imagine a speaker with a valve for a cone, it could work.

My first thought is that a valve might be way too heavy to move with the sort of frequency that a small speaker cone can.

This is completely barring my insanely outlandish idea of transferring motor power via induction coils and motors rather than a crankshaft...