Yep - concur here. If the house uses conventional wooden joists and floorboards, or sheet material, even if you build a solid effective freestanding wall, you have one sheet of plasterboard and one sheet of flooring to the cavity - and as the cavity runs to the problem wall - and often into it if the house is older and the joist sit in channels - there is a very easy route for the sound to travel. You'd have to get into both cavities, floor and ceiling and install a barrier. That's not remotely a simple job. You could add more layers to the ceiling, and put a layered floor in - but you'd need to consider loading. 18mm of extra plasterboard is about 30Kg for 1.2 x 2.4m - so that's no lightweight mass you are attaching. Same thing if you add in extra floor layers.