I also am moving my band into the garage and am researching ways to do it. I heard things and all of them are about the same as this.
I built a room-within-a-room on the first floor of a guest house, converting the garage area. Since no one lives on the second floor, I didn’t have to deal with the floor or the ceiling, just the walls. I sealed the garage door off, and caulked it shut. I used a double wall construction with 1” of drywall on each side, and a double door system as described above. I can tell you that I play my drums as loudly as I can at 3 am, and no one’s the wiser. The dB meter registers no change AT ALL. If you listen carefully, you can hear the faintest sound…mostly the bass. Walk about twenty feet away, and it’s gone.