Ok you need to know a few basic elements of sound isolation. Sound can leak through any gaps, and can vibrate through structures. The basic method for sound isolation is to create an air-tight room, with a mass-air-mass structure. Mass-air-mass can be a layer or drywall, an air-gap(as much as possible, maybe filled with insulation), then another sheet of drywall. To reduce structural vibration, it is necessary to build a floating room within a room, with no flanking path for noise.
This is a very basic setup(1st attached pic). This is a good starting point though.
Within the shed, build a floating floor(9'x15'), and frame a room on top of this floor, completely decoupled from the rest of the shed(no flanking path to the rest of the shed). Install standard insulation and drywall, then build another stud frame between this and the rest of the shed, as shown. Install insulation in there, and drywall. Install a solid core door on each of these stud walls(old doors will be fine), with door seals all the way round. Make sure everything is caulked up, and try to run electrics surface(caulking up any holes).
One cheap but noisy way to ventilate is to use in-line fans with flexi duct(one intake and one extract per room). Remember holes mean sound leakage, so you need to work out how to get the vents through the ceiling/walls without any sound leakage(2nd attached pic is one suggestion to be adapted).
Now onto acoustics. You want to get as even a response as possible within this room, and the worst frequencies as bass frequencies, so minimum i suggest is bass traps in the four corners shown, but more corners(wall-ceiling) would be a good idea. You also want to absorb the first reflections, side ceiling and back wall. I'd suggest for a better setup also treating behind the speakers, and more side and ceiling panels.
Within a tracking room, variation is the best option, imo. I'd suggest a few moveable traps, and some ceiling panels. For both rooms, i'd suggest a reflective floor.