Mount the boxes as normal if not clipping then use extension rings and longer screws to attach the outlets/switches. If clipping then you'll need to bring the boxes forward to meet up with the backside of the first layer of drywall. Or you can use adjustable boxes. I just went with extension rings myself as they're about 80 cents each for a single gang from an electrical supplier.
Soundproofingcompany.com has diagrams of how to mount the boxes with some 2x4 blocks so the boxes are sufficiently proud to penetrate the first layer of drywall when it's a clipped wall.