I have had silencers for both the M4 and the pistols. (As well the 22 pistol, Amphibia S which has a built in silencer.) They do not seem to be working.
The M4 and pistol silencer do not mask the sound in any way. And any shots fired still attract Zombies.
The Amphibia S silenced gun does have a muffled sound. However, it stills pulls zombies as if the gun made a "normal" firing sound.