Hi Mike,
In general, the length of your run will determine the gauge required. How far is it from your amp to your speakers? Unless you have a run more than, say, 50 feet, I'm pretty sure that 14 gauge will work.
How much is it actually going to be outside and in the elements? If it's not susceptible to lots of sun and weather (e.g., mounted to the underside of a deck or under an overhang/roof), I would probably just go with some 12 or 14 gauge from Home Depot.
I believe that the in-wall rated stuff is called CL-2 and it's also available at Home Depot/Lowe's. I'm not sure the difference between it and the "normal" stuff, but it may be more resilient to those types of effects (it may also be a fire-retardant thing that makes it "CL-2"). I'd also check that out.
Anyway, a couple thoughts...