The sheet rock is already up so this will be fun.
Ugh, that makes for quite a hassle. Do you have to undergo an electrical inspection for your permitting process? I imagine if your drywall is already up it is past that point. The reason I ask is that building code in most area's require specific types of cabling be used inside of walls.
The Stereophile article mentioned included Home Depot extension cord used as speaker wire. It actually sounds pretty good (tried some out for the heck of it). But that stuff is pretty beefy to force feed through your studs so I personally wouldn't go there. It'd sure be cheap though. Goerz makes some flat cable that is supposedly code for in-wall wiring. Don't know if it would be any easier to route, nor how good it is. A 50-foot run may require some specific considerations for running a signal that distance.
Good luck!
Marco