The first thing that I would suggest is framing a wood floor over the concrete. While the concrete is certainly a solid base to anchor to it is also more than likely pitched to provide drainage which could be annoying for setting TT's up and so on. Most are pitched anywhere from 1/8 to 1/4" per foot, if you have a level garage floor punch your contractor the next time you see him. Also, most garages are 6 to 8" below the main house floor so this would allow you to bring it up level with the rest of the house. The false floor also would provide a nice chase space for wiring, plumbing, etc. so that you don't have to get into walls or run stuff over floors. This is the time to do things right so plan ahead and save future headaches. I'd also think about framing double walls around the perimeter of the room (staggered studs or otherwise) to provide isolation from the main house and limit sound transmission. I can provide more details if you're curious.
Those are my first couple thoughts, I'll post more if the appropriate synapses fire.