Would this work? Soundproof a party wall. On an already plastered wall (Dot and Dab) drill 1 hole every 1ft, then fill with Expanding foam. Cover wall with MDF 1/2" or 3/4" the thicker the better. Use silicone adhesive to stick the MDF to the existing wall, being sure to fill all the gaps with silicone. Then get some soundboard plasterboard and do the same to the MDF, skim and paint....Job Done!