Originally Posted by
spikerS
The idea of a cold air return is to pull cold air in to cycle through the furnace. Cold air is always at floor level, so putting the return at the top near the ceiling, you are drawing out the hot air, leaving the cool air behind. so, no, that doesn't really work, and you lose probably like 75% of your efficiency.
To insulate as much as possible of the wall, I would add in 2 more studs to frame in the return as much as possible, and then insulate those voids. It looks like you are trying to sound proof the room, so I get why you want as much insulation as you can, but, short of relocating the return to a different wall and location, that's what you get.