14 gauge should be sufficient I would think. You can use any type of cable really if it is in conduit and going under your yard.
They also have direct burial outdoor speaker cable if you want, then you can ditch using the conduit. I would use a flexible burial conduit, plastic, not metal. The plastic won't corrode, will be flexible, and will keep all moisture out. Of course, if you aren't burying the cables, you really don't need conduit, just a straight cable run under your deck from inside your home to the speakers.