Cost effective way to absorb sound in a wall unit?

I have an in wall entertainment center unit and I need to kill (totally absorb) the sound bouncing around inside part of this cabinet. I know it is not "allowed", but, I have two speakers inside this in-wall unit. Yikes! I know I'm going to get it from you guys. Anyway, I have no other place to put the speakers. The dimensions that I need to deaden are 81 inches wide by 40 inches tall. Side walls are 40 inches tall by 13 inches deep. Roof and floor is 13 inches deep by 81 inches wide I was thinking about some kind of polurethane foam to glue on the walls. Foam under the speakers and to the side of speakers where they just barely touch the side walls. The area will be hidden by overlay fabric so I don't can what the sound abodrber will look like. Their is a big TV in the middle of all this. Referrals to specific places and products would be appreciated. Some say wedge foam is better than pyramid foam. Please help