Every one has a smartphone that can play sounds, has a flashlight, can vibrate and display images or colors on the screen. That is exactly what we used, in random order, based on the music and video on the big screens.
Based on the time of a webserver, the countdown clock in the app were synced, when you installed the app. So everybody will get the same experience. On the big screen there was also a countdown. No internet connection is required during the show.
Flashlights blinking on the beat of the music.