I used sound board (similar to homasote)... I did not glue/screw or otherwise attach it to the plywood. Track and accessory screws are only secured to the sound board (not through to the plywood). It's dead quiet. If you run a screw through or glue whatever insulating surface you install to the plywood substrate you will lose a good deal (if not most) of the sound deadening attributes.
I found it a little easier to work with (not much) than homasote. Here in the south it's primarily made from sugar cane (the leftover waste after processing the sugar).
