Here are some topics you need to discuss with your contractor, there is a good chance he will either stare at you like a deer in the headlights, or tell you none of this is necessary. You need to be prepared to educate an uneducated contractor on theater building techniques and not give in to the "well I've never done it that way, TRUST ME, my way will work"
Sound isolation requirements
for the space and construction methods to be used to acheive your goals. these may include
Isolated framing methods
Clips and channel isolation
Extra massive and damped drywall, two layers of 5/8 drywall with Green Glue for example
Sound Attenuating duct work and isolation from the rest of the house
Beefy door with perimeter seals
All wall and ceiling penetrations (Lights, wall switches, outlets etc) addressed with backer boxes or Putty pads.
Stages, Soffits and riser built after drywall is in place
Pre wiring requirements
speakers, lighting, projector etc, use of conduit for at least the projector signal feeds (cable standards change over the years)
Sealed sound isolated Theaters are high load areas and must have dedicated supplies and returns to handle th load. The velocity of air through a vent should not exceed a velocity of 250 ft per min this usually means over-sized vents for both the supply and returns. Too small and they will be noisey.
Interior finishes and acoustical treatments