Any suggestions for improving this weak point?

There may not be anything I can do that is reasonable, but it's worth a shot asking... I have to soffit around some duct work in my basement studio. The return duct goes to the room above here. I don't have an air-tight room yet so it's hard to tell how bad it will be in the end, but as of now, this is a definite weak spot in the isolation. The first picture is a drawing to visualize the whole problem. The second picture is a view of the returns from the upstairs room. The third picture is looking in the return from the upstairs. It runs between the joists and then straight down into the return duct. The fourth picture is looking up from the basement. There isn't a lot of room to get some mass against the bottom of the joists here... but maybe that will help?