It depends. If you are shooting supersonic there’s nothing you can do. So first think to do is choose pellet heavy enaugh and make sure you are subsonic.
After that there’s lots of things you can do. With simple frame-extender and some baffles you can reduce almost all sound of pressurised air. Then you have the sound of hammer slap which is like closing the car door. You can silence that too. But at the end you have the sound of pellet hitting target. That’s the hardest one to quiet down and exactly the thing I try to figure out currently myself.
Also the environment plays a very big role. In my backyard there are brick walls in almost every direction and they reflect the sound making it sound louder than it is. If I go to field it feels many times more silent.