Go to a hardware center, buy some cork tiles (thicker the better), use construction adhesive to attach them to the offending wall. Do all of the wall, don't leave any areas uncovered ('cept for electrical plugs and other fixtures of course). Worked for me in my office.
Soundproofing???????????
i just moved into an apartment, no one below me so they wont hear my music. BUT the people to the right (wall) can hear everything.and i can hear them talk & yell. whats the easiest & cheapest way to fix this?? the walls seemed pretty solid when i checked them befor moving in but i guess there not soundproof..help