Answer:
That depends on many factors. To sound proof a room, mass is your friend.
Most commonly used material is Standard drywall.
Some high end material like Quiet-rock has more mass and damping capabilities and provides a higher STC.
A layer of drywall or quiet-rock on the walls of the auditorium should provide good sound proofing.
Foam, fabric and white noise can also be helpful when dealing with a building previously constructed, with poor sound proofing.