I've been researching speaker wire as well, in-wall and in-room. The website from Roger Russell, above, is great, very informative, but doesn't speak directly about in-wall.
I emailed Mr. Russell, and he said he's been using standard 12 gauge in his studio for in-wall for some time. Good enough for me, so that's my plan. I will probably look for something with thick insullation - my attic gets fairly hot during Texas summers - but other than that, 12 gauge should more than meet my needs.
Hope this helps. - Paul M.