It builds up beacause it can't get out of the theatre. The actors talk. We can still hear what they said a minute ago. This happens in any closed room - sound never leaves it. If someone goes out to get some soda or popcorn.
Some conversations escape as they leave and enter the lobby.
When the movie's over. And everyone leaves the theatre. The film is a gift to the surrounding community. In the future everyone will have the same haircut and the same clothes. In the future everyone will be very fat from the stachy diet.