How would the acoustics be with the rough texture inside? Also - could you only listen to rock music on those?
Rough textures are good. Smooth, flat surfaces cause standing waves (echoes basically) that can color sound. Concrete is a pretty tame surface, though. There are speaker companies that try to make the inside of the cabinet more like an anechoic chamber where there are no parrallel surfaces at all. (Some of the B&W DM series speakers were this way, but I can't find any images of it for some reason.)
Concrete is definitely the right material for stopping vibration, though. It's very rigid, and heavy too. There are a lot of people who will cast cabinets for subwoofers. I also remember a couple builders who'd made a double wall wooden speaker cabinet with a cavity in the middle that they'd fill with concrete.