When I ran CO2 and a 16" Dye Boomstick, I regularly got asked if my gun was shooting right even by people in the same bunker with me. It was without question the quietest set-up I've ever run. From the front, you'd hear the ball go past as loudly as the shot.
The 16" AA barrel I have bored out for Freak inserts seems to be just as quiet.
I'd agree with everyone else -porting and CO2.
Also, if you're playing woodsball and getting lit up after one shot regularly, that means you've probably either shot too quickly (wait until they're so close a branch isn't likely to be in the way) or you miss a lot. You can fix both of those holes and be sneaky even with a gun with a loud report.
My advice, don't shoot unless you can't miss. If you have to shoot when they're further away, try to time the single shot to line up with an increase in firing around the rest of the field and to occur when the person isn't looking in your direction.
If you shoot at people when they are looking in your direction and before they are so close that the ball will definitely get there, your problem is not how loudly your marker fires.