It is super easy. I have 2 supply ducts and now a return duct in my theater. ZERO sound transmits through the duct. How? Just used simple flexible, insulated ducting for the ducts in the room. There is still a little rigid duct between the theater room and the rest of the basement, but it is mounted up in the floor joists and has normal fiberglass insulation between it and the ceiling of the theater. It definitely takes more that a couple of inches though. You need some good length and at least one 180 degree bend (don't squish the duct though). The sound bounces into the side wall of the insulation at that point and basically destroys its ability to continue down the duct.
The people at The Soundproofing Company have diagrams buried in their website somewhere, but I know from actually doing this that it worked. I have a YouTube video from my construction where I talk about it and you can see it. Unfortunately, I am being summoned to go bowling like *right now* with the in-laws.
Typical Christmas get together activity.
I will look it up later, or look for my "I should start a build thread" or whatever I called it topic in this Home Theater part of the forums. Towards the last few pages, I list the videos and give a brief description. It would be from before all of the drywall is done.
