I put the carpet padding between the studs 4 layers thick, then covered the walls in sheetrock, then hang thick black cloth, pleated on the walls. As far as sound isolation, it's like this, we were getting the Police called on us alot from neighbors a half mile away. After remodeling the building and putting the carpet padding in the walls, we play louder, and you can walk to the end of my driveway, and you can't hear us at all now. The only sound that gets out now is the bass amp, and it's not very much. When I add some bass traps this problem should get solved. And the Police haven't been to my studio in a year now. It's not as good as rockwool, but no where near the cost, in fact, a carpet store gave us all the padding for free just for going to where they were taking carpet up, and hauling it off. Worked well for all concerned.
Terry