Add another layer to the walls. You want to insulate, literally, all sound in that room from sounds that could creep in, and the sounds within that you don't want to creep out.
Think of sound as if it were water. It will find ways to go places that you can't find.
This is very true. You have to confine the sound if you are to feel the full impact. I guess that without a good surround, then the quality of sound will be hugely compromised and nothing is as bad as a good movie with poor sound quality.