Re: What material to stuff inside drywall cavbity?
If absolute sound isolation is your goal you're going to want to jump on-line to check out what insulation brands R ratings are going to give you the most sound dampening for your money. That being said, be careful not to make your room to "dead" acoustically.
the thick padding might work well. Perhaps doubling up your subfloor would be better. You could even 'float' a subfloor.