I helped my brother build his recording studio and some of the things we incorporated are as follows:
It was a room inside a room. The outside wall was insulated with Corning 703 insulation. There was an air gap between the outside studs and inside wall. The inside wall insulated with 703.
Everything had double sheetrock with all air gaps filled with acoustic calk including the framing where it met the floor and ceiling and all walls.
The ceiling was double sheet rock. It's very important to "float" the ceiling using channel strips designed for this purpose. It you attach directly into the rafters or joists the sound will directly transmit to whatever is above regardless of the number of sheetrock layers used. This is critical.
All venting was insulated. We built collector boxes in the venting runs which were nothing more that a square box lined with 703 insulation.