It's cheaper and more common than I thought. Google "audio over cat5" or "audio over Ethernet". The main question will be whether the solution is asking for a direct connection between your devices or whether it works through the normal IP infrastructure of routers, hubs & switches, I guess.
Apple's "AirTunes" via AirPort is very easy to set up but it normally requires iTunes to be the source. This can be circumvented by "Airfoil" (available for Mac & Windows), a virtual router that can send anything to the AirPort. From there, analogue or optical digital audio can be taken direct. Of course, this solution requires that you can get your live mixer output into the Mac/PC somehow so that it can be seen by Airfoil. For this reason it sounds to me like the most expensive, unless you already have the means to get the digital audio into the computer.