I buried 1 1/2" PVC conduit and pulled standard audio cabling through it. One huge benefit with conduit is the ability to re-pull lines if anything ever goes wrong or if upgrades are required (as long as you leave a string in it).
Terminate the conduit into
one of these with a
flip-down weatherproof cover installed beside the speaker location (you could T your conduit to run lines to multiple speakers in different locations). Finally, install a pair of
banana plug jacks and use short links to connect your speakers (you'll likely need to hunt around for a keystone plate that fits the box/cover). As these links would be short and accessible, use whatever cable you have available as you can easily replace them if they become damaged.
I use this method to provide line-level audio, CAT-3, Ethernet, and coax to outdoor locations, and everything has been operating flawlessly for over a decade (and counting).