Look for sound transparent fabrics that are aesthetically pleasing. The fiberglass panel underneath is what matters.

Owens Corning fiberglass panels are what you need. See the link below. Build some simple frames to hold them. Wrap them in acoustically transparent fabric of your choice. Hang them in the appropriate spots. Voilą! Better sound immediately.

Amazon.com: Owens Corning 703 24"x48"x2" Fiberglass Boards - Pack of 6: Musical Instruments