Smartphone plan? They don't have to be activated to work. A Droid A855 is $15 shipped on eBay. Load it up with MP3s and you've got an excellent media source. Grab some BT speakers (or a BT rx + speakers), and you're good to go. At least, I was.
I took an old surround system and linked it to an A855 with BT. Built reflectors into the corners of my office between the bench and cabinets for L/R channels, and put the bass through a reflex cabinet near the back center. Feels like the music seeps out of the walls in my office, while you can't really hear it next door. And the phone can be in my pocket for control of the music as I walk around. Interface is robust for playlist generation, and customizable for what the external buttons do (e.g. long press Vol Up skips to skip forward, long down skip back, etc).