I bought a single wireless 'weatherproof' speaker from Wal-Mart a couple years back. 65 bucks. The receiver for it is plugged into my receiver in the house (50' away), and does just fine. I didn't want to get too sophisticated with running wires, but not ruling that out in the future. Wireless speakers don't last forever. If it does get wet, like when I forget it and the sprinklers go off, then it needs to dry for a couple days so it will stop crackling.
I think the #1 thing you need to worry about with electronics outdoors is humidity. A receiver will turn into a rusty mess if they can't breathe.