I'm talking about the phone receiving a broadcast. The app has to be designed efficiently such that it minimizes network activity and doesn't hog the input audi

I'm talking about the phone receiving a broadcast. The app has to be designed efficiently such that it minimizes network activity and doesn't hog the input audio stream. You also have the issue of trying to maximize the probability that a device in proximity to a beacon will pick up the full message. So you need some redundancy, which obviously increases overhead. NTP sounds like it would work. As you kindly noted, just make sure the beacons broadcast at a agreed upon time and in fixed increments.