I'd imagine it's faster and cheaper to buy a bunch of Squeezeboxes. Though you could also try running Squeezeplay or Squeezeslave on the Pi boards, all pulling from the same server, which you could also run on one of those same Pis. Trying to reinvent the wheel in this case could be a huge hassle, unless of course you want to do it for the challenge or to create a product of your own.