Malibu wire for low voltage outdoor lights works great for speaker wire ... I have 4 speakers in my yard using the malibu wire to power them. I use 2 recievers in the house so I have seperate volume control for inside / outside.

Speakers are Bose 151 Enviromental (10 years outside in the sun, rain and sprinklers and they still sound great) out in the yard and klipsch for under teh patio.

try the malibu wire, you don't even need conduit in the ground then.