We have an HVAC system in our house where the duct work is all sheet metal with no insulation on the interior of the ducts as all of it is buried under inches of foam roofing insulation.
Works great. Efficient air flow with great insulation performance.
The problem is that the pipes carry sound -- conversations, TV, etc.. -- from some vents to other vents. Not very loud, but loud enough to be annoying.
Is there some way to deaden the sound transmission without grossly reducing airflow?
And, of course, do so keeping in mind that all of the duct work is completely inaccessible?