air is the best soundproofing (analbumcover...great name and great advice). the more air space between your old wall and yer floating room the better.
i cheaply made my place pretty soundproof by putting a sheet of that pink 2" compressed styrofoam housing insulation covered with thick eggshell mattress foam. i keep all my speakers a couple of feet from the walls as well. it isn't proper (the real sound proofing sheets are 200 bucks for a 2x2 and i have no space for a floating room) but i haven't had any complaints. but i think my neighbours may be too afraid of me to complain. hmmmm maybe they can hear after all...

i-ron