I'm asking this question on all 3 forums, cuz I cannot find the answer anywhere. I can find that the Silencer cuts the range in half, but I don't know the number to halve.

You halve whatever figure your GM dictates. On open quiet ground even a silenced shot would still be audible at a distance, where as in noisy surroundings you'd likely have to be very close to recognise the sound as anything other than machinery.

It will seriously depend on location, in a city, or other noisy environment the range that you could hear a gunshot could range wildly from a few yards, if you were in a construction site, to half a kilometer in a relatively quiet area. In a rural area you could reasonably expect to hear gunfire from about 3-5 kilometers, again varying heavily on ambient noise levels. Of course determine the direction of fire, or even if the noise you just heard was gunfire or something else is a whole other problem.

Of course that really only applies to conventional firearms, las weapons make a low cracking noise like a mini thunder strike, and I think boltguns make a low crack and then a hiss as the rocket ignites. I can't really imagine what a plasma, or melta weapon would actually sound like