Be careful with the 12 foot ceiling dimension relative to the other two of the rectangular box. Understand the difference between sound isolation (soundproofing) and acoustics. Isolation is a science, acoustics an art. Only thing to add about isolation is close up every hole and gap with acoustic sealant and put seals on the door because that’s where the sound will escape. And don’t be surprised if the low frequencies still get out after doing all of the other soundproofing. Design for flexibility in speaker placement and seating position. If your room is 12x12x12 change it. First time post from an architect.
