There are two routes I'm thinking of going: Either representing quiet by illustrating an idiom or using contrast to show a quiet moment.
1. My idiom of choice is, of course, "so quiet you can hear a pin drop." With a large, expansive room and a small pin falling to the floor, I think I could adequately communicate "quiet".
2. The first thought I had for best showing "quiet" is conveying contrast between a quiet and loud place, which emphasis on the quiet space.