I have some pretty expensive equipment inside my house that I am worried about. If for some reason the speaker would fail oh well. However if the problem would come into the house I would really be upset. So either some kind of special wire or some way to block my equiptment from be trashed.
Depends on how long of a run. If it's about 100ft+, I recommend 14gauge, if it's less than 16ga should be fine.
I really like the Monster 4 conductor 16ga direct burial wire. It has a plenum sheath for protection, and string for easy jacket stripping. Expect to pay about $0.90 per foot for it though.
Low voltage lamp wire will work, I don't know what you're priorities are for sound as it will likely change it a bit depending on the speakers. The Parts Express Direct Burial speaker wire is significantly cheaper than the monster stuff and probably works fine.
I like to dig a 12-16" ditch in the ground for low voltage burial wire. This can protect it from a gardner's shovel. I found out that the higher voltage wire requires 24" depth minimum.
Some outdoor manufacturers recommend installing fuses near the loudspeaker to protect the wire.