There is a special industry that concentrates on nothing but soundproofing, and there are many products available to solve different problems.
Soundproofing against human voice transmission is easier than soundproofing against low frequency noise or amplified sounds.
Double walls with special material bewteen are very effective because this reduces noise transmitted through physical structures. This can be done with 2 X 4s placed flat on a wall plate or by staggering 2 X 4s on a 2 X 6 wall plate. Even better if no common plate or structure is shared between the two walls.
Assuming that you are not going to rebuild the walls, there are special wall treatments available.
Here are some materials for residential applications: http://www.asknetwell.com/residential/residential.html
Search on the internet for "noise control" or "sound control" and you will fine many links and products. Then, call or e-mail the companies and discuss your application. Different sound insulation materials have different properties that that are effective for certain applications but not for others. Tell them your application and they will suggest products.
If all else fails then wear earplugs! Good luck! Billy