Professional Audio is the way to go
The threads Allen sent are great. There are so many options. Right now I have centralized audio outside through a Yamaha Console (16 channel) and I use Wav Runners for my MP3s. I love these things (Hauntbot) because they are dependable, cheap, and very simple. I use amplified speakers inside because I like the punch and the flexibility they give me. I really only use an amp for my outside speakers (big Peavey speakers I bought from a rock band).
When its time to turn everything on, its a few switches and I am ready to go. I will be expanding this rig to cover inside as well.