When you sound proof rooms for recording you are basically doing 2 things; you are trying to stop sound from coming in and sound from going out. This is accomplished by sealing doors and windows and any place where sound can come in or go out.
Installing solid core doors, double windows and double wall/ceilings is the best way to accomplish a true sound proof recording room. Using sound silicone weather strip gaskets for doors and windows will accomplish the best results. I also recommend using silicone caulking for all joints including floors and ceiling.