There's a couple ways to do this... not sure which will work best for you.
If the framing is heavy enough, you could attach OSB or MDF to the bottom of the roof deck, between the framing, and then attach drywall to that. Couple concerns here...
First, check on how the roof is built. If it's just metal panels with the rubber roof on top, I would avoid this route because screwing to the metal panel may puncture your waterproofing. There may be rigid foam insulation on top of the metal deck, there may not, but FIND OUT. If the rigid foam is up there, then you should be ok- and most of the time, there will be, but you don't want your studio to be leaking water every time it rains...
Second, if the framing is light (bar joists or something like that) then this won't do too much good because sound will get right between your nw panels and thru the roof.
Anyway, if you don't mind losing the ceiling height, and if you'll be building a room-within-a-room, it may be better for you to just put drywall on the bottom of the framing and build the second leaf up from underneath.
The third thing you can do is kinda what you suggested; since it's the roof, chances are it's pretty much sealed (otherwise it'd leak). Go around and double check on that, but it may function as a mass leaf just as it is. It may not be much of a mass leaf, but I believe you can just make the inner leaf heavier to compensate (Knightfly will correct me if I'm wrong, but I've seen it suggested elsewhere...)
If you're using bar joists, you can probably set your internal room framing between them and have the finish ceiling inside the rooms only lose a little height. But then again, if it slopes up to 18 feet, you've got plenty of height...
Hope this helps!
Kase
www.minemusic.net