If by noise from creaking boards. Only a well secured floor will stop that. Until it ages but then its fair game. Cork wont stop that. It will dampen noise from walking or rolling things across the floor. Wont eliminate it. One thing to consider is if it is a nail down product (the floor) then you need to think about the length of staple or cleat to use. A standard 2" cleat wont secure the floor well enough through cork. That extra height the cork has takes away from the amount of bite the cleat will have on the subfloor. Not a huge issue in terms of price but its an easy oversight that some may not think of.

I think the key word is "floating". This isn't a floating floor, it's a naildown. I don't believe it's going to be nearly as effective in a naildown situation, as most of the noise that is "muffled" in a floating situation is from the fact that there isn't good solid contact with the subflooring, as in nailed down. WHen there is good solid contact with a subfloor, the majority of the noise produced is from the reflective (rather than absorbtive) nature of the hardwood surface itself. Sound just echoes more into the air from a hardwood floor than it does from a sound absorbtive carpet floor. No amount of underlayment is going to alter the nature of the sound echo produced because the flooring is a hard floor product and not a textile based flooring product. It may be possible that installing a manufacture approved for naildown installation sound barrier can help with some of the percussive sound transmission from floor to floor, but it'll never be as quiet downstairs as if carpet on padding was the second floor material. And, you'll still get sound echoes upstairs that travel through the walls and open areas, simply because the wood surface reflects sound. You can't alter the basic nature of a hardwood floor. You can use rugs and additional soft furnishings to moderate the sound absorbtive properties and mitigate some of that reflectivity, but you'd have to practically cover all of the rooms wall to wall with rugs to make it be as quiet as carpet could be.

Either fastening method is fine. If you screw it down use construction screws NOT drywall screws. Although 5/8" or 3/4" is preferred as a subfloor, I have seen many hardwood floors that went over 1/2" plywood that are just fine. I doubt that a 1/4" overlay is going to do much of anything for you, except maybe prevent the fasteners from blowing though the 1/2" subfloor. Is this a new house? And...or...what is the joist spacing and do you have any sag 'between' the joists?

I have a room over a garage and i have used a product called Tredaire Boardwalk.tog rating 0.55 It will only work if the floor is floating. you can download a data sheet from a company called underlay4u if you go the there wood and laminate page Here is a link that might be useful: www.underlay4u.com