There is a "Sound Attenuation" product on the market that you might check out. I don't think it rises to the level of "sound proofing" but it may help. I appears identical to insulation. In fact the drywall installer in my daughter's new house wanted to know why we installed insulation in the interior walls. My sketchy knowledge of "sound proofing" leads me to think that it involves the type of construction rather than an add on material, i.e. offset studs, etc. It seems the idea is to prevent sound transfer through the structure rather than absorbing the sound.