The polystyrene will have no effect on acoustic isolation.
configuration for acoustic isolation is to use a layer of 19mm plasterboard next to the
studs and a second layer of 12mm acoustic plasterboard over that. Keep the joins between
boards staggered between layers. If you want to go even better you can add another layer
of 15mm acoustic plasterboard. On my recent studio build I also used Green Glue between
layers while on the previous build we had a layer of fibreboard (known as a Camden
Your local builder's merchant may well offer better deals on
plasterboard than the big DIY places.
JRP Music - Audio Mastering and Restoration.