The stuff that's mimicked in movies would probably be something like a silenced 22 short round or maybe a subsonic 22LR round. The primary noise of a gun is the bullet breaking the sound barrier, the secondary noise is the actual powder burning. You can absorb some of that with a suppressor but if you shoot a round that goes faster than the speed of sound, you'll still get an audible and identifiable 'crack'
A hand gun round will usually run you about 145-160db. There's a lot of variation here like barrel length, load, all of which influence speed which is also a factor. Take 30db off that with a suppressor and you're still looking at over 100db and maybe even over 120db which is the hearing loss range.
At best, a suppressor brings the sound from 'hearing loss' to 'quite loud' for normal rounds.
For a subsonic 22 round you're looking at something like 60-70db, take 20-30db off that and you get that sound that you hear in movies.
They are no where near as quiet as movies would have you believe. There are a lot of factors that come in to play as to how quiet they are, like if you're using sub-sonic ammo, the size of the supressor, barrel length, what the bullet hits, and more.
Sadly, I'm sure a lot of their regulation has to do with how they're portrayed in movies rather than reality.
A cinema effect, which is why they’re usually called suppressors. It lowers the decibel level of the sound enough that you can fire without ear protection and not risk damage to your hearing.
You're not going to get a centerfire rifle to be "Hollywood quiet", no matter what you do. You can get a .22 rimfire rifle or pistol down to the level that it sounds like an airgun.
... but it sounds like an airgun because it has about the same energy as an airgun.
