I have chosen to build second stud walls on all walls except the wall between the recording room and home theatre and one exterior wall (more on that when I start work on that wall).
Most walls will be attached to each other with WIC clips from Mason Industries (pictured below). For most of my walls, placing these clips at 8ft centres will suffice, with one clip at each end of a wall. I built the walls on the ground first (longest being 16ft), stood them up, beaded the bottom plate with soundproofing caulk, fastened the bottom plate to the concrete with red head bolts and powder driven nails. Then I attached the WICs to each wall with 1 inch lag screws, making sure the new wall was standing square.
I have built a soffit around the HVAC ducts. The underside of the soffit connects to the inner wall directly (with wood screws). The top plate of the soffit connects to the joists with RSIC-DC04 brackets (2ft on centre) to ensure sound transfer to the floor above is minimised.
I did run into a problem with the WICs for some of the walls. There was tension pulling the two joined walls together, which caused the WIC to slip (the glue did not hold). Since these clips are used in music halls and tv studios, I assume it is not a flaw in the clip, but that they are not meant to handle this kind of strain. To prevent the walls pulling together, I used RSIC-DC04s mounted on the inner wall, as a buffer (I wonder how this problem is dealt with in commercial construction).
