OK. Thanks.to all.
The consensus seems to be "an ordinary quilt/tapestry/rug".
I was thinking of the front face of the hearth, rather than the top.
Flexible room treatment material???
My speakers flank a large slate-tile hearth that also houses a glassed-in propane fireplace.
Is there some sort of flexible room treatment material that could be fashioned into a "quilt" and hung from the top of the hearth to cover these hard surfaces while listening???
I’m very happy with the sound I get but can’t help but wonder whether such a strategy might yield worthwhile benefits.
My wife is handy with a sewing machine.