Brad,
With all due respect, I would forget the wireless speaker idea. I doubt you'll even find a wireless speaker that will hold up to the weather. Secondly, all the effort around A/C and wireless signal distribution is going to be a hell of a lot more work than running good speaker wire no? Check with the town, perhaps there is pole to pole conduit run underground you could use? If not, can you span wire rope taught from pole to pole and then attach the speaker wire to it? Sort of like a tight rope? Lastly, there are plenty of good sounding and extremely durable outdoor professional speakers to choose from - and will offer you far more options than anything wireless.
ScottH