Fantastic blank canvas!
That room is in a bad spot for sound proofing, but you can substantially reduce the volume by:
Installing a floating floor
Installing thick insulation in the walls, before covering them with plaster - extra points if it's again covered with something like further panelling, or acoustic foam etc.
Treating the ceiling as well - many forget this step but it's important
Covering and sealing all windows
Sealing the entire room - any cracks or openings will allow a scary amount of sound out.
From here you have an uncomfortably sealed room with no oxygen and no personality, but it'll certainly deaden a good amount of sound.
Next, make it fit for humans - ventilation is a hard case, you want to let air in and out, but not sound. In my room, I built two boxes about 3 or 4 feet high, that contained a labyrinth made of timber directing the air to zigzag one way, then another, from the bottom to the top of the box. With openings at the top and bottom, these were then installed in the ceiling cavity, with the bottom openings poking through the ceiling, on opposite sides of the room, and the top openings ducted away from the neighbours. Next, I installed a centrifical fan which draws air from another room in the house ( thereby bringing in the heat and oxygen from the house) via a couple of filters, and ducted it to push air though one of the boxes. It then exhausts out the other box, giving me clean quiet air flow.
I'd also recommend separate power to this room from the rest of the house. Impossible totally of course, but an improvement in interactions with other electrical a in the house can be had by running separate power cables back to the fuse box for this room.
The final touch in making it fit for humans is to make it an enjoyable place to be ... Lighting that can be dimmed without inducing dimmer noise in the electrics, colours, posters, motorcycles... All the good stuff. Funky leather couches, huge studio monitors and enough equipment to make you wonder how the hell can you ever get bored.
Great project. Please keep us posted.
Thanks
Pauly
C