This particular gun is in a safe condition, with the hammer down on an empty chamber. It appears to have been converted to single action.

Probably a convenience thing.

The beretta 92 combat, unlike the og 92, has a frame mounted safety that, when safed, physically blocks the slide from moving--same as a 1911.

If I had to get a pistol ready for an actual white-box photoshoot, I'm gonna clean the shit out of it, which means full disassembly and scrubbing all the little fiddly bits. Can't take it apart with the safety on, 'cause the slide would be blocked. Probably wouldn't flip the safety on after I got it all dry and back together because, yay it's photo time.

Alternative possibility, same reasoning: I store pistols with chamber flags in, so I can be sure there's not a round in the chamber. This necessitates slightly open slides, and safeties that interfere with the slide can't be kept on in that state.

Not sure, I haven't really noticed this. I guess it makes it look more action-y or something?

I feel like a lot of the rifle pictures I see tend to have the safety on though, especially AKs (where the safety is very visible). I don't pay as much attention to pistols.

No 92 I have shot or handled has been perceptibly louder than any other semi.

Oh, it isn't mine (I'm from a country where there isn't really much gun ownership), I just have lots of pics saved.

I would guess the noisiness of the slide is related to how fast it travels (and thus how hard it slams into the frame), which is a function of its weight and the energy of the ammunition. Not sure why a slide would be uniquely noisy though, unless the gun is being shot with some real high pressure ammo or something.