If you've ever seen the t.v. show 'Holmes on Homes', there is an episode about a duplex and the families on both sides can hear each other clearly through the walls.
In one episode (Wall of Sound), they use an insulation called 'Roxul Safe and Sound'. It's sold in Canada, but I'm not sure about the U.S. Also, they off-set the 2x6 studded wall. Then they used a special kind of 'drywall' that has 10x the sound-deadening of regular drywall. It's made of a special polymer and has a thin sheet of metal that runs through it. Before they started and when they were all done with the sound-proofing, they had a guy come in with a decibal meter. The meter proved that the sound-proofing was extremely effective. You might be able to go to www.Holmesonhomes.com
to find out the names of the products and where/if they can be purchased here.