Try hanging some carpet strips on the wall. Find some carpet someone is throwing out and try some 2' wide strips the full height of the wall. Space 2' apart. See if that helps. If it does, you can get some carpet and frame it with wood molding to dress it up. If not, try mounting them perpendicular to the wall. Other possibility, though more expensive, is Homosote to cover the walls with. I'd check with the manufacturer to make sure it would work for that application though. Then there is the material that recording studios use on the wall to absorb sound. Can't imagine what that would cost. Jim.
Home of Irish Setter Rescue of North Texas
When Irish Eyes are smiling, they're usually up to something!!
At a minimum, I'm Pentatoxic...but most likely, I'm a Pentaholic. There seems to be no known cure. Pentatonix, winners of The Sing Off, season 3