This is a concept for an installation art piece which I won’t be building. But I like the idea.
In a reasonably sized, but not too big, room, say about 10×10 meters, you install infrared motion detectors, hanging from the roof. They have to be directional, meaning they should only detect motion in a narrow beam, emitting from the detector. Like those motion detectors you see in old spy movies (Think Pink Panther).
The motion detectors are hooked up to a sound machine (or a computer) and, when detecting motion, trigger the playing of a sound clip. Not just a note, but a sample, ranging from under a second to a few seconds or maybe even longer.
Then, fill the room with people and have them walk around. They’ll trigger a piece of music which is practically impossible to recreate.