Most sound from outside the house comes through the windows rather than the walls. Internal sound transmission between rooms is usually the biggest issue but can be improved dramatically by making sure that there are no air gaps at the top or bottom of any partition. Filling the void in the wall with insulation or sound deadening material can help as can additional layers of plasterboard. Sound transmission through floors can be alleviated by filling the void with mineral wool or by the use of specialist screeds.