Because you are running it in wall, (and because I'm a fan of their cables) - I might suggest the Canare speaker wire. http://www.canare.com
The 4s8 is 4x16 gauge wire that is equivalent to 13 gauge wire, because you use each twisted pair as you would a single conductor.
I say this might be good for in-wall, because this wire twisting method is excellent for RFI rejection... and there can be plenty of that in your wall, with power lines and whatnot.
You can get this for about 40 cents a foot from www.markertek.com, so it's not pricey by any means.
