It's an unofficial version based off 1.2/1.3 that adds, among other things, a silencer to the pistol that can be toggled with secondary fire.
You could make a new model and replace the sound, couldn't you? I think the muzzleflash is placed at an attachement, so you'd just need to move that.
But NPCs would still hear it...Although, they don't really notice it much anyway, so it might work.