I have the silencer in my mod, and the problem is the ai doesn't react to seeing other dead bodies, so basically you can pick all the ai off one by one and they won't detect a thing, even though they are within whispering range.
It's quite simple to actually code though, just change the gun volume to quiet, remove any references to the silencer code, and make the silencer a part of the main weapon .mdl, not optional.
You will need to change the sounds, the attachment needs to be moved forward to accommodate the new position of the barrel, but really it's rather simple, assuming you have any modeling, coding and sound editing skills.
Unless you want to make it optional, in which case, good luck.