Underlayment – What Type of Underlayment Is Best for Metal Roofing?

Per request, I'll expand my comment into an answer:

as for the joints in the concrete, I'd use polymeric sand (typically used for locking pavers).

Then the key is to create some sort of smooth pad in between the concrete and the bottom of the pool. Some options:

• roofing felt (tar paper).
• Ice and Shield roofing membrane
• rubber pond liner
• epoxy (if it's not terribly rough, a few coats of epoxy concrete sealer might smooth it out)

You probably missed "the best option for reducing noise to below" if you put down new subfloors and didn't put insulation under them (assuming you actually removed the old subfloor and had open joists.) Your next-best option would be to blow in insulation from below.

If you are already committed to nail-down flooring, I don't think the underlayment will make much difference, as the nails will transmit sound into the structure. An actual floating floor on the thin, dense foam underlayment sold for that purpose would be the best option on top of the subfloor for traffic sound reduction - or a "no shoes on the second floor" house rule.