chip,
I built my walls off the existing floor on the concrete slab in my garage. I did put down a rubber material along the entire length of the bottom plate, so the wall would not touch the concrete (this is something that I thought up, not in any of the books)
Then I built my floating floor inside the new room and it does not touch any of the walls.
DISCLAIMER: This is my first studio and I am still in construction, so I do not have any performance test of my resulting work. But a lot of my thinking and ideas came from F. Alton Everests books. (I think thats right) But I have made some mistakes already, but I am learning.
That's pretty bad when the disclaimer is longer than the reply. lol
Larry