Do you have a smartphone/tablet? That would probably be the easiest route, as your smartphone/tablet should easily get a wifi signal from your home. From there you can use all the usual apps (pandora, spotify, etc) to google music/itunes, or if you want to play mp3s directly off your PC, there are many file browsers available that you can explore your entire computer network with.
From there, all you need outside is any set of speakers that plug in via a headphone-type jack if you want to get off fairly cheap, or if you want the luxury of sitting with your phone/tablet in your hand and the speaker not near you or attached via cord, there are a lot of bluetooth speaker options. Something like this would work nicely: http://reviews.cnet.com/bluetooth-sp...-35567062.html