The idea is that as the spectator walks past a wall mounted object the sound is generated, if they move on the sound stops.
Forget about there being 3 of the things. If the idea is that the display makes noise when a person approaches, and gets quiet when the person leaves, the experience is not the same if there is another person approaching while one person is stopped.
If you define what you want to happen in this situation, the implementation of the whole project is trivial. It is defining what to do when there is more than one person at/near/approaching/leaving an exhibit at one time that is the difficult part.