Don't forget to insulate the ceiling. I've seen people go to a lot of trouble to separate rooms and then use a drop ceiling with the open area sharing the space above the rooms.
Also rooms sharing vents and returns will allow sound to travel through them a long ways. The duct and return system acts like a resonator. Use a door sweep under any doors and weather strip around the doors if you are really paranoid about someone hearing your clients' deepest, darkest sex secrets.