An old timer carpenter once told me that a simple inexpensive to soundproof( to what percentage I can't say)yet used this in our inside walls to our bathroom and found it suitable, was; to line up glass bottles (not touching one another of course) in the interior studding between the wall.This may require removing the sheetrock of one wall or so but I extend this plan to you with sincereity.Oh by the way it was fun and we added small cross pieces of studding to accommodate more bottles.
