[Reaganesque]

You could add piezo-electic materials to the speakers. Vibration causes them to produce electricity. Someone has already proposed putting such materials underneath roads so that everyday traffic would generate electricity for cities. They would lay it down as part of the road building/repairing process so, it wouldn’t cost all that much more.

[Arthur McGowan]

You could add piezo-electic materials to the speakers. Vibration causes them to produce electricity.

Now you're getting into perpetual-motion territory.