Noiseproofing Bedroom with Green Glue: Should I do exterior walls and/or ceiling?
It depends of how much noise you would like to reduce. if you looking from complete silence, applying sound-proofing sealant to two walls will do nothing but empty your wallet. If you want to reduce it to a level where you can read, write, or just not be bothered by it; then sound proofing two walls and the ceiling will help. The theory of sound proofing is to eliminate or reduce sound vibrations traveling through the walls. This is achieved by creating a air space between the layers of sheetrock where vibrations go to die. When you screw it to the wall, the metal of the screws will act as vibration transmitters and diminishing the effectiveness of the soundproofing glue. You will be better off by glueing it to the old wall.

If you have access to the room where the noise is generated, you can reduce the noise by sound-treating that room with sound absorbing furnishing and diffusing sound traveling by  rearranging furniture, etc. A quick search for sound proofing art will bring an array of artsy and decorative items that serve that purpose.