Create a gallery wall of flat, polyester fiber sound-absorbing panels (the BXI Sound Absorber panels are $19.95 on Amazon!) This material has been engineered to absorb a wider range of frequencies ...

Add 1-inch-thick acoustic panels. Sheetrock, a terrible absorber of sound, can be the culprit of that tinnyness you hear when talking on the phone. To remedy this, cover your walls with materials ...