For standard walls sure. But i'm building a theater room and sound proofing by double walling it(room within a room) Going to turn out great
Again over kill. You really need to decouple the Gypsum from the studs, but first off, the studs for the walls need to be properly built, since they will be the weakest link. As for sound through any duct work, there is no way to stop that, unless you use the proper materials, and they have to be fire rated, if you line the ducts.
Also no need to crank the volume up so high, that people think that you have Motorhead playing in your house. The loudest that we turn our system up to, is maybe -12db, which is enough for us. Most times, it is maybe at -18db for about everything, including movies.