Yes, ventilation for the roof is only for thermal concerns.
For best STC rating don't drywall the underside of the rafters if you are building a room inside.
Be sure to fill the airspace with fluffy stuff. - And put a plastic moisture barrier toward the room (before you put up the drywall). If you can seal up the small cracks and holes in the exterior wall with a non-hardening caulk it would be a bit better.
I am currently designing a similar project studio in Australia.
It's your call on escape windows and/or doors, but they will reduce your transmission loss if not built correctly or sealed properly.
It is the internal room that must be sealed completely.
Cheers,
John