You are talking about the large cavity behind the box, right? Not how the front beauty board conforms to everything? If this is the case then I think people are misunderstanding your question. It took me a minute to figure it out. Besides walling it off again behind the box I would think the only thing you could do would be stuff it full of insulation to deaden that part and force more of it forward.isn't it losing pressure back there because its not sealed off? kinda like sealing off the trunk of a car
Now keep in mind that I am not a professional and know diddly squat about walls.