In my build, I have one long wall of existing wood 2x4 framing. The doorway will be in this wall. That wall has drywall on the outside, nothing inside. To the theater side, another 2x4 steel framed wall will be built, open to the side facing the existing framing, and DD+GG inside. To keep these two walls decoupled, what should I do at the doorway, and at the end of the double wall, where it turns the corner to make a perpendicular wall that is just a single wall? Leave it open there? Extend the single wall drywall all the way to cover the ends of both walls? How about at the doorway--if the jamb spans the width, will I ruin the decoupling?

Also, the ductwork ends up right at this double/single wall junction as well--as long as there are a couple layers of drywall in between this ductwork and the room, can it just be an open(fiberglass-filled) space as it exits the wall junction? It will be in a soffit along the long wall, and goes straight where the wall turns 90 degrees. You can see it here--the front wall will go to your right about 2/3 way through the duct length. Long wall to the left, not yet a double wall in this pic.

http://img.photobucket.com/albums/v613/Kevin_5/Home%20Theater/614817ac.jpg

How about at the doorway--if the jamb spans the width, will I ruin the decoupling?
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While I don't have this problem I've read of 3 solutions to this problem.
In Descending decoupling preference (I'm guessing here).

1) double doors (communicating doors) one mounted to the outside wall, one to the inside leaving an air gap between. The two jambs are mounted with a slight gap and the gap filled with a rubber gasket. Or two doors with one thick solid jamb

2) single door, a split jamb with a gap and rubber gasket

3) single door and an extra wide jamb spanning both walls.

In any case a good solid door with weather stripping and a threshold sweep seal would be required to match the performance of the wall you are building.

Can't do double doors here--no room to open one inside. So, it looks like #2 or #3, or a combination perhaps. Maybe do #3 with some rubber gasket behind, or #2 with the smallest of gaps between the 2 jamb pieces. Hmmmmmm....

You know, I find what takes the most time in this theater construction game is thinking about solutions, NOT the actual work!

Here is a terrible Paint version of the double wall, soffit, and perpendicular wall to kind of illustrate what I mean with the ductwork and soffit. I guess since it is outside the finished space that you just stuff the area around it with fiberglass and leave it open to the backside?
http://img.photobucket.com/albums/v613/Kevin_5/Home%20Theater/5d7c5e9b.jpg

I replaced a number of metal ducts in my basement ceiling with the insulated Flex duct to minimize the transfer of sound via the HVAC.