We have recently moved and I was able to get the perfect spot for my HT. I am having trouble deciding on the best soundproofing options for the space. The room is above the garage and adjacent to two bedrooms. It is 19’w x23’l (pics are below). Around the perimeter of the room I had a wall constructed that is 18” wide. I did this for A) soundproofing B) ease of access. My current plan is to float the floor and double drywall with GG. I also had to attach 1x4’s to build out the wall due to a beam. Any suggestions would be helpful. I think I have been obsessing way too much on this part of my project and I really need to move on.

Oen can never obsess too much! :)

Sounds like you have a good plan.

What will you be doing about the ceiling in that room? looks like it you leave it vaulted you may have acoustic problems to deal with.
and don't forget the sound treatments for your electical boxes, lights and HVAC.

I am just getting proposals now for my HT. I am planning on using double drywall with GG also. Are you using two different thicknesses of the drywall? Someone suggested that you use 5/8" and 1/2". Also I am going use good insulation inside the walls and use sound isolation clips. I have been told the clips help keep the low frequenies from bleeding outside the room.

Good luck with your HT.

you are in close to the same boat as me...attached structure could be your worst enemy...