Installing subfloor and hardwood on top of concrete (2nd story)
I'd go with either a layer of cork or polyiso rigid foam.  Then a floating floor on top.  A nearby residential tower actually requires the cork underlay.  The rigid foam will work just as well at lower cost.

The float floor can be two layers of ply staggered and screwed to each other, or a engineered hardwood product.

See also the "Sound Isolation Store" for great technical details.