its got to do with how much oxygen is in the suppressor. as you shoot and keep shooting supressors get quieter as the oxygen is used up there is nothing to keep the combustion going outside the barrel so the gun gets quieter. at least thats my understanding. i could be wrong.
but they do get quieter as you continue to shoot without stopping. if you pause for a bit they get a little louder again.
"Gun control: The theory that a woman found dead in an alley, raped and strangled with her panty hose, is somehow morally superior to a woman explaining to police how her attacker got that fatal bullet wound." L. Neil Smith
The problem with being stupid is you cannot simply decide to stop doing dumb things...
"I crapped my pants to avoid the draft!!" -Ted Nugent