Hardwood floors in condominiums

Hi, I'm about to take possession of a condo and one thing I plan on doing is laying down hardwood. Condo policy is that there needs to be a sound insulation barrier. I'm trying to figure out whether I need a plywood barrier or do I go boards onto the cork directly. THis is what they did in my parents' unit for instance, and I think that the floor planks were simply glued to a cork underpad. The other way would be to put cork down, then plywood, then nail the boards to the plywood.
Question is, what are the advantages/disadvantages of each approach? If not using the plywood, and just gluing a floor down was hands down cheapest, then everyone would be doing that, but some people nail a hardwood floor to plywood.
THanx in advance -Hugh
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