Adding mass to a subfloor - am I about to do something stupid?
If it was me I'd properly install multiple layers of a sound deadening insulation between the floor joists and either reinstall, or install new subflooring. This will give you nearly a foot of sound deadening insulation to absorb any noises, plus you won't have to add all of that additional weight. I think that the insulation (can't recall the exact name) comes in 6 or 8 foot lengths, so you can pull up the subfloor a couple of sheets at a time, insulate, and reinstall the sheeting. Just make sure that you stagger the end joints in the insulation and sheeting.
FYI, if you decide that you must add more layers of subfloor then you'll want to run the new sheeting in the opposite direction that the base subfloor is running, and keep the "soundproofing" sheeting roughly 1/2-3/4 away from the wall framing. You also want to keep the sheets gapped away from each other by that same distance. Also, you can fasten the sheeting to the subfloor, just make sure that you don't hit any joists. Yes, I said don't hit any joists. This is all per national guidelines, feel free to check your local building codes regarding sound proofing between floors or a residential structure.
Also, if you were going to gap the floating OSB (you have to gap to get any real sound mitigating benefit, ie. won't transfer sound vibrations throughout the entire floor) and use the 1/2" concrete board to hold it together, or hold it in place, you're going to have some serious problems if you decide to install carpet and properly power stretch in the rooms. I'd bet money that the edges of your floating substrate will want to lift, and the expanding and contracting, as well as factoring in a floating substrate in general, will probably give you more frequent issues with your carpet wrinkling prematurely.
Anyway, just some food for thought. Good luck.
Last edited by RWCustom; 11-02-2012 at 02:36 PM.