I've seen a common product use 10' CONCRETE SLAB + 1' flooring + 12' drop down Amrstrong Acoustic CEILING + 5/8' drywall + Safe&Sound insulation between the drop ceiling and the real ceiling. Companies that sell flooring underlay will PAD THE NUMBERS by ADDING STUFF to the test to achieve MASSIVE numbers. IIC/STC ratings on most underlay = BS = bogus.

And I've heard of this happening with cork and some artificial products. Some chemicals react by creating an odour when the come in contact with each other. And sometimes (just sometimes) you can create an odd odour. This is the most common complaint from people who add underlayment to underlayment. It's like the sound of rubbing two balloons together. Besides, cork and IXPE make a HORRIBLE rubbing sound. I'm sorry but your choice means you cannot use extra underlay. And the maximum thickness it allows (attached or added) is 1.5mm. It is not ALLOWED underpad if it already has it attached to the back. This is the single greatest drawback with vinyl. The moment you have a snick more movement (bounce from a nice squishy product) you run the risk of the edge breaking and your floor coming apart at the seams. That means the click-edge is SUPER thin and VERY delicate.

The 1.5mm cork underlayment is as thick as vinyl can handle. As soon as you use it you void the warranty.

Mass loaded vinyl underlayment