Gs5556 has given you some good advice. I'd stay with the concrete floor; there are many products on the market for leveling your existing floor (Ardex feather-finish is one I've used alot, and is a great product). If you are'nt able to do this, then find a contractor who can. Then I would just go ahead and carpet/pad over the concrete. My sound room in my basement has this arrangement, and the sound is great.
The thought about later selling your home and needing to convert this room back to a garage should be a strong consideration to stay with the concrete.
The thought about later selling your home and needing to convert this room back to a garage should be a strong consideration to stay with the concrete.