I wish my little studio was bigger and more comfortable as it would have to be the best room of it's kind I have been in at anyone's home... well, for my ears anyway.
The walls were filled with cotton/wool blend sound insulation (recovered from a demolished commercial recording studio) before they were sheeted up then the inside is lined with convoluted foam, most of which is actually audio grade. The floor is concrete slab and above the ceiling is one layer of sound wool and then two layers of R4 fiberglass insulation. The room is *almost* dead now. I still have a small part of the the ceiling to line and I will be doing that any day now but as it is, you clap your hands in there and there's no discernible reverb. I have lined the door but it's a standard interior door and I will be replacing that with an exterior solid door as well. The door is the one point of failure for sound penetration.
Originally it was designed as a cool, quiet place for me to escape the kids and study or work. The bonus was noise form the inside also didn't bother the rest of the house so I could work in there late at night (I periodically do on-call work) with some music playing. When I started to get the bug to record again, after being away from making music for over ten years, the room just needed lining with something to deaden the sound.
Anyway, it's a great little escape for me and the sound in there, originally with the JBL monitors and now with the Behringers, is pretty damned good. The Mirages, however, will remain in the lounge area, with all of it's glass and concrete.
EDIT: just by the way, I have seen this product demonstrated:
http://www.acoustica.com.au/quietwave.html...if you are building a room for sound and want to both limit the sound escaping and remove disturbances from outside, this is the stuff! It really is amazing!