Really ? I always thought the sound of the gun was bundle to the weapon itself. I tried in the past to give the suppressor sound to MP40 and fail.
It is- guns can only make the sounds assigned to them.
However, you can spawn the other gun underneath the soldier and have it play the sound.
For instance, you can make an Enfield launch rifle grenades, but it's silent, so you can spawn a K98k to make the sound.
I doubt that would work for automatic weapons, but I haven't actually tried yet, so maybe?