that combines a nasty piezo-electric buzzer with a mini microphone and a little time circuit.
Mike senses loud stereo noise, triggers piezo, which shrieks for a few seconds, goes off, waits a second, if it hears noise again, goes off again, etc.
Best would be to flip it thru an open window so it rolls under the seat, or you could stick it with a magnet.
Guy turns on his stereo -- SHRIEK -- SHRIEK -- SHRIEK -- "WTF?" -- turns it off -- nuttin -- turns it on -- SHRIEK . . . . . .
Just dreamin, of course . . .