Thanks for the comments...this helps.
Overall -- combining the comments from the three of you, it sounds like the idea of "deadening" any sound WITHIN the cavity (to keep any "reflected" sound from making it into the room) should be my focus.
Spaceman, you are correct -- I was talking about the insulation within the stud spaces of the "cavity," or "enclosure," and just leaving it "un-drywalled" inside the enclosure. If you can picture this -- there is open attic behind my screen wall. So, what I did is I went around behind the screen wall, in the attic, and framed large "boxes" out of 2x4s, to serve as the "speaker enclosure." I installed R-13 insulation within the stud cavities of the framed "enclosure," and then on the outside of the enclosure, facing into the attic, I lined it with 1/2" foam board. Thus, from the attic side, all you see are "boxes" protruding into the attic from the back of the screen wall, enclosed by foam board and the seams sealed with expanding foam.
From INSIDE the theater room, you see a screen wall with open cavities on either side of the future screen location, and one centered above the future screen location. There is a "floor" or "base" in the cavity made of plywood, for the speaker to sit on, and then all you see looking into the cavities is the "kraft face" of the R-13 fiberglass insulation that is installed in the stud cavities.
I am not worried about "fire rating," from an "inspection" perspective, since here, in my state, the work I've done does not require permits or inspections. So, if the only reason to put any drywall in the cavities would be to "pass an inspection," I don't think it's a concern. From the feedback given, I'm leaning toward just leaving the speaker "cavities" un-drywalled, and proceeding -- as BIGmouth suggested. I could easily place more fiberglass insulation within the cavities, once I have set the speakers in -- if necessary.
Any additional thoughts?
(I will gladly take a few pics and post them, BIGmouth, if it's still not clear after my above description...)