I have plaster walls and there's no need to put up sheet rock. Use carpeting or oriental rugs, hang drapes, and tapestries, and use soft furniture and bookcases to dampen the room. You'll need to use acoustic panels on the walls to treat the room for sound absorption and/or diffusion. Sometimes a panel is needed on the ceiling.
Look at the GIK Acoustic website for samples and advice. If the room is for listening only, you can use DIY foam panels; not as attractive but inexpensive.
Look at the GIK Acoustic website for samples and advice. If the room is for listening only, you can use DIY foam panels; not as attractive but inexpensive.