When you're crouch walking, while wearing boots, you produce two bars of sound, but when you raise your firearm you produce 3. When you're barefoot crouch walking, you produce 1 and when you raise a firearm you produce 2.

This doesn't make any sense and seems to be a bug. It doesn't allow players to hear you better, because it doesn't seem to increase the sound only the sound bars in the hud.

I have noticed that it increases the range at which infected aggro on you. If you are sneaking up behind one with a gun, and then raise your gun while moving, they will immediately aggro.