sound proofing an interior wall

sound proofing is best with a denser material in my opinion. Also note that creating air gaps helps with sound reduction. So creating an air gap between the drywall, your sound insulation material, and again another air gap between the other side of the walls drywall would be most efficient. If you're using a sound barrior type board, this can be done by packing out on either side with 1x3's. On interior walls in my experience most use bat insulation for sound deadening, as it's the easiest and most cost effective. However, they do make specific materials for sound deadening, if you want to foot the bill.