IF you were going to run a drive in movie with modern tech, how would you handle sound?

Three options present themselves to me, and I'm not sure I like any of them:

- Big 'ol PA system (too loud for the neighbors, not loud enough for the cars)
- an array of smaller speakers hooked over each car window (I have no idea what kind of equipment I would need for this)
- AM/FM transmitter (cars have to be running the whole time, or we risk killing folks' batteries.)