Simple question, need simple answer

I want to put some layers, on the backside (inside surface) of some drywall, in an interior wall. I've got a couple of inches of available thickness to work with, between studs. The remaining thickness is taken up by an air conditioning return (the AC can be turned off, whenever needed, for quiet). What is the single best type of layered structure/material I can put, in between the studs, on the back of a drywall? I'm guessing 3/4" MDF, with some type of soft/insulating material between the drywall and the MDF. Such as a few blocks of styrofoam, to hold the MDF in position, with thick fiberglass stuffed in between the styrofoam. And then caulk the MDF edges to the studs. Can anyone suggest something better than that? I'm not trying to become an expert; I just want to deaden a few feet of wall.

Simple answer is that anything you place in between the mass that does not have mass...like Styrofoam, will be detrimental to your isolation. Having no more to go on than the little bit you present, I assume you are doing what is termed "beefing up" a panel. In that respect, add mass, not fluff...it depletes you air and reduces the mass ability to stop sound by adding multiple separations.