As far as insulation, have you priced foam, where it is sprayed into the walls? Acoustically I would think that would be better than fiberglass.Foam won't deaden/damp the Gyprock (drywall) panels like compressed 2 x 4" batts will, as the cavity is 4" you have 8" of batt compressed into 4" that will damp/deaden both side of the dry wall.
Cheers George