How to best tie multiple background music systems together?

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I have large building that is really 3 old building tied together.  Wiring from one building to another is difficult, not impossible, but very difficult.
The goal is to have the background music in all building playing the same thing (from one source).   Currently there are speakers in each building and each "building's speakers" go to an old stereo tuner of some sort amplifying the signal.  

We have a "Music on Hold" device that allows music and "ads" to be loaded onto a USB and this device has a simply RCA Out jack that feeds into one of the tuners.
We have standard 14ga SPEAKER WIRE running from the center-most tuner to the other 2 tuners.  If looking at the attached PDF, we have speaker wire running from A to B, and from A to C.
If that wire can be used to connect them together that would be AWESOME.  If I need to run a different kind of wire because of new equipment needed, so be it.
I have attached a rough diagram that hopefully will make clearer my situation.

My question is what device(s) can I get to make all these speakers play from one source?   Either new amplifiers, or something that could go from an output of one amplifier to an input of another.   AND.... of course I'm trying to do this as inexpensively as possible.  
Thans for any help.AudioDiagram.pdf
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Dr. KlahnPrincipal Software Engineer

Run the following goofle searches:

"network distributed audio"
"audio over ethernet"
"audio over IP"

You will want to either dedicate its own cabling / fiber to this application, or buy a product which compresses the audio before shipping it.  "1 Mbit/sec is typical for uncompressed audio."

It is much to your advantage that you already have the audio plant in place, because some of the equipment is ... well, not expensive, but not inexpensive either.  Example:
Top Expert 2016
since you have tuners with amplifiers  you could use an FM Trasmitter or you could use your existing wiring  since you have 14 ga wire ggoing to all 3 tuner locations

first floor - rca jacks line out -> 2nd floor line in and 2nd floor line out to 3rd floor line in.
On the 2nd floor you might need to press the monitor switch.


I will try that. Thanks.

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