Thanks for all the tips. Of course, I was going for the completely lazy solution, but the acoustic panels do look nice and don't look that hard to whip up. The rear wall does have a spot for a panel; and I could lobby for heavier curtains all around.

Has anyone every done anything clever with a stone fireplace that is never used?

Also, the question was explorative, I may not need anything; I have never had speakers in this room so I may get lucky. Someone had mentioned a "clap test" somewhere along the line and when I did that I got nervous with the results. We'll see soon; the three weeks are up and I'm waiting on good shipping news from Axiom.