I have seen several products to control sound transmission through an exposed radiant slab on plywood to the living space below. The idea is to reduce sound from "impact", for example a dropped marble or footsteps. The underfloor insulation etc along with the concrete should do a good job on airborne sound. Some of the products resemble carpet underlay (offgassing ?) others rainscreen netting. (Keene makes rainscreen mats and sound control mats) anybody have experience with any of these products?

Cork works good

Have a lot trouble with marbles?

One has to be careful to avoid getting the sound attenuating material between the radiant panel and the room it serves. High-density fiberglass is the most cost effective sound deadening material that will usually serve well as the requisite insulator below a radiant panel.

Though cork is a wonderful flooring material, with excellent sound attenuation, durability, water proofing and unfortunately a great insulator. I have a cork floor in my own bathroom but did the math so the the floor PEX tubing below (with extruded aluminum plates) has a chance to drive heat into the room and satisfy the heat load in the coldest Minnesota weather.

Any cementious material over PEX, plywood and sub-floor insulation will be reasonably quiet, but the first order of business is the final surface floor covering. This process starts with a proper heat load analysis, which dictates pipe, insulation, design water temperature and most importantly final foor covering.

Regupol is another good sound attenuation product