1/4 inch drywall is available just about anywere where rock is sold. Its main purpose is to overlay old rock that is damaged beyond repairs via tape and bed work.
I use some last week on a den area that was previously paneled. In stead of using nails and friendly glue, these people dumped tubes and tubes of liquid nails on the rock.
I could not even budge this globs of stuff.
So, I just saved time and gave the room a new set of walls. 1/4 inch is soo soo easy to handle and work with. You do have to be careful because a slight bow will snap it. But this is also easy on existing electric sockets, window trim, base boards and door trim.
Now, the sound board is very hard to find for me. When I found it once it eas almost $25 a sheet. It is used in sound music rooms. Many bands want a room done in this for accoustical purposes.