To stop the transmission of sound through the various electrical boxes which will pierce the drywall shell, I used "firestop." I got this stuff at the local electrical supply house. It is kind of like a very thick putty or playdough, but very sticky.
It might be hard to tell from this shot, but the electrical box is one whose depth is adjustable after it is nailed to the stud. This is necessary for my application, as I can't be exactly sure how far out the resilient channel will flex. With this box, once the last layer of drywall is in place, it is simply a matter of setting the depth using the adjustment screw. With the firestop around the back and the acoustical caulk around the perimeter of the box, the air seal will be maintained during adjustment.
