Having seen videos of a suppressed MP-5, they make a rapid clicking noise on full auto. It’s still loud by my estimation but it doesn’t sound like a gun.
What you're hearing is camera attenuation. They are still very loud, they just won't outright blow your ears out. It might not sound like a gun if you don't know what a suppressor sounds like, but you will still be able to hear it unless the noise of the environment is hearing-damaging loud. Just about the only things that are Hollywood-quiet are subsonic suppressed .22s and .45s, but the former is incredibly anemic and the latter is not cheap.
You should be able to easily find on YouTube/Reddit or other parts of the internet- actual decibel testing.
For the CP33, with some of the higher quality suppressors, the action of the gun is actually louder than the fired munitions. I haven’t checked on things in a bit, but people were trying to work on the action to reduce noise created by it.
It’s essentially slightly louder than a handheld staple gun irl (there’s videos of this comparison on YouTube) - and I’ve saw somewhat reputably claimed, but not actually demonstrated - there are now some integral aftermarket barrels for the Ruger MKIV that are quieter than the CP33.
As you’ve stated though - these rounds definitely don’t have much power behind them and will do practically nothing to anything that isn’t human flesh like. They can quite literally bounce off of an obese person or get caught in a modest cold weather coat.