I personally wouldn't use it except for perimeter boards around a floated floor. From what I've read about actual tests, a thinner middle layer of sheet rock works just as well if not better. I've not used the homasote in a sandwich, but if doing a floating floor with minimal headroom it could be an alternative to rubber and joists - just use a layer of homasote under the floor layer. Not as effective as rubber, but better than nothing.
I wouldn't use it anywhere you wouldn't use particle board though, it's crumbly enough it would absorb moisture.
As to adhesives, when you're doing multiple layers in each leaf of a wall you should avoid adhesives. You want each layer in the leaf to act independently, so that there are different resonances and weak spots. That way, the whole is more effective than it would be if all layers were acting as one big mass.
The only exception to that is if you're putting multiple layers directly on studs, then it's slightly beneficial to use a stripe of glue over the studs and fewer screws (16" spacing instead of 12") - the glue makes up for the fewer screws, and the fewer screws decreases flanking noise that gets through by following the fastener into the frame.
Hope that helps... Steve