‘Shhh’.

Theatre John Wyver attends Noises Off and ironically has difficulty hearing the show due to the incessant talking from the audience:

"To the Old Vic to sit with Clare in two eye-wateringly expensive seats to watch an immaculate performance of Michael Frayn’s Noises Off. The back-stage comic complications, combined with the high-end prices (top whack £85 a seat – that’s eighty-five pounds!), have attracted an audience that is well-heeled, well-dressed… and well rude. I am used to people trying to talk through movies, and I have extensive experience of tapping shoulders or turning round and emitting an urgent, audible ‘Shhh’. But in the theatre?"

Now I know it's not just me.