I have a friend who spends a lot of time with a household name musician. The guy just had a studio built in the basement of his house and he had the walls soundproofed. I saw the room while it was under construction and the framed walls were filled with insulation that looked like shredded blue jeans or some other cotton-like fiber. The walls were double sheetrocked with 5/8" rock with a layer of material in between that looked and felt like the stuff used in wetsuits. They also floated the floor over the concrete slab with some sort of rubbery sheet between the slab and 1" MDF decking. I'm not sure what they did as far as the ceiling goes, but I'm told that you can beat on a guitar with a 200W marshall amp in there and you can't hear a peep outside of the studio space. It must have cost a pretty penny though.

Cheers,