The fault of sound coming through a party wall generally does not lie so much in the wall itself as in the joists. It will probably be found that the joists rest in the party wall, possibly touching each other, and that the sound is conveyed by the timbers, not by the brickwork. The skirting boards, too, may be acting as Bounding boards. If this is the case, "jack up" the end of each joist, take out the brick below the end of it. and insert a thinner brick, with two layers of tarred felt between the brick and the joist, at the same time wedging a piece of felt between those joists that touch each other. The skirtings should be taken off, and the space behind filled with plaster. If the cause is really in the walls and not in the joists, try covering with one of the thick pulp papers, such as Lincrusta-Walton, anaglypta, or Japanese leather paper.