If you are serious about a floor that performs at the highest level, then install two layers of plywood with Green Glue in between and 3/8" Serena™ Underlay to create a floated floor system.
In real life applications, this floor system performs better than any poured gypsum floor at a fraction of the cost.
Add Green Glue + Double 3/4" OSB + 3/8" Serena® Underlay
Note that nail-down hardwood flooring requires a nail base of two sheets of plywood. These two layers are cinched together with short screws. These screws cannot be so long as to penetrate the rubber layer. The finished hardwood floor and nail-base are “floating” on the Serena Mat®.