Looking for the best easy way to get some sound proofing in an already built bonus room for HT? Would replacing dry wall with quiet rock work?
No, it will not "work" unless your idea of "working" is a 1-2% total gain, if that.
We need much more information - and "soundproofing" (you will want to look up the term "isolation" for what you need) is not a possible goal / doesn't define what you need.
Where is the sound leaking to that needs to be stopped? What is the wall construction? second story of a house? Basement? HVAC runs? Are the doors sealed? Do electrical boxes need to be isolated? How much isolation do you need, at what frequency levels?
There is no "hang this on the wall" soundproofing solution. It will likely require construction. If you need just a tiny gain, adding a second layer of drywall with greenglue could add some benefit, though I expect that isn't what is causing problems with sound transmission.
