What you want to achieve is best managed by using UPnP. But that cannotThe answer does not exactly matches what you asked but it shows how it may be installed just one time ondone with media standards and the option to build it up step by step, e.g. playing same music in many rooms using a deviceRaspberry Pi and control it worksfrom your pocket.
You have to setup a system that consists of a Media Server, some (at least one) Media Renderer and a Control Point. With this components you can attach a media renderer to music boxes for output. With the control point you can manage what music getting from the media server is to play on the media renderer. You can also select what media renderer to use.
For example I use a built-in UPnP media server on my internet router (an additional goodie).
As control point I use an App on my android phone, named DroidUPnP. But there are some other Apps usable as control point, e.g. BubbleUpNP. But I don't like it because its a heavy weight and do much more than I need (only control the media renderer).
And finally to come to Raspberry Pi you can use it as a Media Renderer. How to setup it you can look at Howto install DLNA media renderer.