Another business plan.
Design a door with non-noise producing hinges, with a knob that doesn't make a sound, and with some kind of latch that doesn't make a loud snapping noise when you haven't pulled the door completely shut, even when it seems like you have.
As an added bonus it wouldn't have a large gap underneath which lets lots of sound into the room. Doesn't have to be sound proof, just noise-reducing.
Please, oh wise entrepreneurs scouring the internet for ideas: take this idea, and make the door, and I'll buy some. Lots of them.