The silencer will change the report of the gun but not by much. They are foam filled and you will get a muffled sound. Usually the other sounds (mechbox, motor) are louder than the bb exciting the barrel anyway.
For certain players with very silent setups in sniper rifles (sorbo pads, soundproof material in their recievers, quiet motors, or even bolt action, etc.) the silencer can do a great deal to hide your position.