Paint a sound-reducing paint on the walls. This type of paint, such as Acousti-Coat, reduces the amount of noise by 30 percent and is easily applied to most surfaces. You can have it tinted at the paint store when you buy it; however, it only takes light colors.
Cover the walls in fabric to mute the sound. Use heavy curtains and wall upholstery to cover the walls. Carpeting up the walls will also mute the sound.
Build large bookcases or similar structures on the wall and attach a soundproof fiberboard, such as Homasote, to the back. This type of board is available at home-improvement stores.
Stick cork tiles to the wall. These are available with self-adhesive backs and are easily applied. Use them as is, or cover them with wallpaper.
Hang sound-proofing wallpaper. This specially designed wallpaper is fabric based, so it has more soundproofing properties than regular wallpaper. It's installed the same way as regular wallpaper.