A really interesting project here. I got asked to bump a wall out to expand a closet and sound proof two walls in a home theatre. What I did was a combination of methods to do the best I can. A double wall, insulation, a soundproofing rubber mat, two layers of 5/8 drywall, and a soundproofing glue between the layers of drywall. It worked really well from the testing I was able to do. I also built the closet out, including a custom built drawer bank. Overall I am super happy with how things turned out.
