I know this is probably not what you want to hear, but if it becomes a serious issue you could switch to broadcasting your sound and have your movie viewers bring a portable radio. Since there radio is right next to them they have a tendency to keep the volume lower. And if someone wants it really loud there are always head phones. Switching to broadcasting my sound made my neighbors much happier. And after 8 years of doing it that way I don't miss messing with the speaker one bit.
I'm so glad we have a large lot and understanding neighbors. A big part of our outdoor theater is the sound. We usually run Dolby/DTS decoded 3.1 channels (left, right, center and sub). Occasionally we run rear surrounds for a full 5.1 sound field, but not too often. I'd be really sad if we couldn't run our sound rig.
Our "concession stand" is a ways off from where we show movies. I've thought about transmitting FM stereo in addition to the 3.1 sound. That way we could have a radio running in the concession stand so folks could at least hear the movie while they're getting a snack. I have an FM transmitter I bought from movielover a few years back. I've not used it yet! Maybe this year...