When you say 8 inch ducts running thru the room do you mean the vents for the room or vents thru the joists above that go to other rooms? If they are for other rooms then they should not pass thru your sound reduction layers. If they are for the vents in the room then you need to build a dead vent. [Link: soundproofingcompany.com] or google dead vent. Also never use the word soundproof to a client, the most you can do is sound reduction. True Soundproofing only happens in uber expensive commercial environments that are way beyond the scope of residential construction.