I'm not here to hustle any products or anything, but I was quite taken by these two websites:
Quiet Solution and Homasote.
Both talk of their soundproofing capabilities--apparently one layer of the Quiet Solution stuff soundproofs as well as eight sheets of sheet rock. (Oh the studio owners I have known thinking they were whipping it out with three layers of sheet rock.)
Homasote is a 'green' product that has some distinct advantages over plywood.
anyhow, back to rammed earth...