Sound dissipates by the inverse square law, so usually what you, the photographer with the camera right next to your head, will hear will be exponentially louder than what others around you hear. Also, the amount of ambient noise and reflective properties of the room will have a big impact on what you can get away with. So for instance, if you're in an empty concrete room with just one or two other people and no air conditioner or other white noise source, just about any camera will be too loud. And if you're in a crowded bar packed with drunk people and furniture, then no camera is going to be heard. So camera choice isn't that big of an issue, since the difference between a loud shutter and a quiet shutter will be dwarfed by all of the other factors. Besides, what's more distracting than the sound is the sight of the cameraperson moving around (or flashes!). Most of time if you don't see the camera, you don't notice the sound (unless the room is dead silent). But if you see the camera person, then you'll think every click you hear is coming from them.