Drywall: 2nd layer on attic floor. Glue or no glue?

So I'm putting a 2nd layer of drywall on my attic floor which also acts as my first room's ceiling. (I've already put up the first layer of drywall, obviously). There will be a total of 3 layers on this floor (2 layers of 1/2" drywall and 1 layer of 1/2" ply on top). My question is this.....would you suggest that I glue any of these layers together? (I.e. glue the drywall layers together with liquid nails or maybe glue the plywood to the 2nd layer of drywall?) I seem to remember reading somewhere on here that you get more "soundproof-ness" out of individual layers as opposed to huge, monolithic glued-together layers but I want to make sure I'm not making things up. Don't have the budget for green glue, unfortunately. Thanks.

I don't believe that gluing two layers of drywall together will help soundproof any more than the two layers simply setting on top of each other. Green Glue is not a normal adhesive. It stays "rubbery" and flexible after it drys. Other "glues" will not help in a similar manner for soundproofing. Using liquid nails for sound isolation is a waste of money.