Great warn, Mike, soundproofing means reflecting sounds back to its origin (keeping the sound energy inside the room, too) and, in other hand, acoustic treatment implies in getting (at least part of) this energy off the room. Never solve sound leakage adding pourous materials (foams and fibers) over a wall, they just absorb highs (while letting lows spreading inside and leaking to outside).
To isolate you need massive walls (gypsum drywall or mansonry). To increase its efficience you can double the walls, but avoid them to contact each other, since lows transmits stongly by structure (mechanical transmission). This is the idea behind box-in-box concept, decoupling walls. This can be improved decoupling floor (with rubber/neoprene cushion) and roof (held with hangers). It is a good idea to base your inner wall uppon sound bands to prevent vibration transmissions, too.
To guarantee air in the hollow between walls to not ressonate, you should use porous material to reduce the free space, though. Inside the room, foams are usefull to attenuate reflections and reduce reverberation. But as they act on lows mainly you must match low absorption (bass traps, corner traps, etc) with care. That is why it needs carefull calculus and dimensioning.
Rehearsal rooms are much more simple to build then Recording spaces. Studios need more care in Isolation and control over lows. And although mixing rooms doesn't need to deep isolation, they need even more carefull sound treatment (since you mix based on what you hear and room's reflections can colour and change anything produced by your monitors). So, positioning, room geometry, room proportions, surface treatment (absorption, diffusion, reflection?) are all ultimate concerns if you want a flat and transparent room...
