No sound, as it does not travel far enough and is easily disrupted by any other sounds in the area of the detector.
I think it is just waiting for input of an electro-magnetical wave, then timestamps it and sends it to the server. The server can calculate the position with just the difference in arrival time to each station, and the position of those stations.
There is a project to detect lightnings over Moscow with cheap sound-only detectors on the roofs of skyscrapers, they managed to restore ever shapes of lightings in 3D with it:
