I am installing a nail down 3/4 inch solid wood flooring over 3/4 inch plywood over cement on my ground floor. I would like to use cork underlayment for warmth, floor evening, and sound softening.
Must the cork be glued or may I staple it? Should I use 1/8 or 1/4 inch cork? Should I install felt over the cork for ease of installment?

just let it lie...no fastening necessary...not much a difference between 1/8 amd 1/4...not that you will be able to note..

but obviously 1/4 probably slightly better for what you want to do..

vapor barrier under plywood first..over plywood..then cork

use 11/2 " nails staples

Why not lay the cork under the plywood and then have a floating install of the hardwood on ply over the cork. Make sure to use the moisture barrier before cork.

I'm with Mickey on the floater! No sense in fastening to concrete it's just an avenue for potential problems. But I hate ripping that 3/4 into 16 inch strips. Go two layers of 3/8 or 1/2 if you can and run them opposite directions, glue an fasten together.

I appreciate all of the responses. I already have the nail down 3/4 wood floor ordered. The sub floor is installed. I just need to know whether installing both the cork and the felt is a good idea. Thanks.

I don't see any reason to use both.