Wood – Cork on _top_ of hardwood floors

Yes indeed, prep is required and very important. First step is to renail or screw down the plywood subfloor. If you have a pneumatic frame nailer, you can use 2 1/2 inch threaded, ring nails (never use common nails), or alternately use 1 5/8 inch drywall screws. Install your nails or screws every 8 to 10 inches apart along each joist line with special attention to seams of the plywood. The importance of securing the subfloor well cannot be understated. The major cause of squeaky floors is drying or aging wood loosening around the nails, allowing them to move.

The second step is to lay a layer of building felt or rosin paper between the subfloor and new hardwood. Simply roll it out and staple it enough to hold it in place. This will assure that there is no wood on wood contact which can also result in squeaks.

Sealed cork is definitely durable. My parents' house has cork in the kitchen and the family room which was installed ~25 years ago, and the only place where it's showing some wear is in the spot in the kitchen where my mother drops things regularly and they've shifted the oven and dishwasher in and out for servicing. Knives do stick in it rather than bouncing off, which may affect the permeability of the sealing.