A suppressor even on air guns would prove to just slow the air down coming out of the barrel, the same way it does with gun powder propelled projectiles.
Slowing down the air will lesson the sound of the air forcefully leaving the barrel.
Muffler.
On firearms with no moving action, as to one that puts in a new round into the chamber semi-auto, if you can reduce the the force of the air coming out of the firearm enough, you would hear no extra sound at all from that, it could be completely silent.
Especially, if your projectile never goes super-sonic.
The sound of a projectile whizzing by your ear will always remain, but that's not a sound one can really hear unless the projectile comes close to you.
Its all about how slow you can get the air coming out of the barrel.
Sure a trigger pull may make a little noise, even a hammer dropping.
With ammo with primers, those primers may make a small noise too.
I've personally used and heard suppressors on 9mm handguns, shooting sub-sonic ammo, that were so quiet they were literally a whisper. Right conditions as well. All you could really make out was the semi-auto action of the slide throwing in a new round.
The light noise was a "Teh-kit" sound. That was all. You could go even quieter with an air rifle with the right suppressor, and it being single shot with no moving parts putting in a new round, and no primer being struck by a hammer.