The silencer is an upgrade for the hand-gun or the Sub-Machine Gun. It's exactly what it sounds like: it mutes the gun shots so other monsters can't hear you shooting their partner to death. But there's more: it also eliminates the flash of the initial shot. You can find the silencer hidden in the newly plastered wall in the same room where you find the mattress. One of the walls is lighter than the others. Hit it with the pipe, a couple of time, and it should crumble, dispensing the silencer and a pair of legs.