For AEG:
Well, there isn't much of a sound reduction no matter what you do; you can use sorbo pads on the piston head which will dampen the sound.. a little... Next you can fill the body will foam but i doubt there will be any space for that anyway.
You can put a mock suppressor filled with foam.... but... that'll next to nothing because the sound is coming from the gearbox.
The best thing you can do is shimming the gearbox, use good quality grease, correct basically everything and it will reduce sound.... slightly.
Next, you can use a higher voltage battery like an 11100V lipo so the gearbox sound won't last any longer than a millisecond
For gas:
Tm mk23, silent already