 Okay, so the best insulation to use for that is mineral wool insulation. Owens Corning makes a product that is called Thermafiber, and it has very good sound-resistant qualities to it. You'd have to open up the walls to do this. You'd have to take off the drywall, and then you'd put the Thermafiber in. Now, in addition to the Thermafiber, there's also a sound-deadening drywall you can use because you have to tear off the existing drywall.