Sound Isolation / 2-car Garage Becomes Rehearsal Space

I'm in Berkeley, Ca. pdf's of the plans are posted here: http://home.pacbell.net/needhame/clark.pdf Db reading of bands that will play in the finished space is 85 db, A-weighted, slow setting. My budget is about 23 k. The contractor I am working with can get the work in the plans done for that amount, but my budget doesn't allow for major demo & rebuild of the existing garage. I don't have that option. I am trying to find out what can be done to isolate sound in the EXTERIOR of this room-within-room assembly. In other words, the garage itself. We'll be putting up 1/2" ply on the exterior of the building, and 7/8" stucco over that. Someone from the Recording.com Forum has ID'd the weak point as the roof. And I'm sure - once we properly frame & install a Rod Gervais-style window - that he's right. The roof is 1/2" ply with comp shingles over the top. We will insulate roof & walls, both, on the inside. But I can see sun shining through seams where the roof joins the walls. Caulking those seams won't add mass. (Space calls this a room-within-a-shed & that communicates the problem rather well) I'd rather not strip off my recently-installed comp shingle roof to add more mass there. It seems to me the weakest link is where the roof joins my walls (that's where I see the sun). Should I screw in blocks around the perimeter, where the roof joins the walls, before insulating? Should I create an inside ceiling above the existing joists, even though that creates a third leaf? Should I add mass to the inside of the roof with sheets of sheetrock?