I recently had the opportunity to pre and post test a room that had been treated with the CSR Soundchek plaster system which involves hanging plaster on channels supported by a rubber insulation system. It’s a very easy system to use and reasonably economical. While it won’t completely soundproof a room with high SPL’s, such as a drum room, it certainly reduces transmission of sound significantly. http://www.gyprock.com.au/resources/filemanager/datasheets.aspx?fileIteration=155
The alternative to this system would be to use Green Glue with plasterboard.