If you're talking about the pressed paper or cardboard type, yes, a bit. The main sound deadening of these, however, comes about in two ways. First, the spaces on the back side allow less transmission than a flat surface. Secondly, the irregular shape allows more sound reflection and also reflects the sound "toward itself", like a dish, thereby cancelling out some of the sound.

I realize that's a snippet of an answer, but I think it gets the main points across.