MusicServer4Lox
MusicServer4Lox is a free/opensource/etc (status unknown) system developed by LoxForum member 'hismastersvoice' (real name unknown!).
The system is developed in German and runs on the Ubuntu Linux Distribution. It is designed to mimic/emulate the official Loxone Music Server closely to provide multi-room audio using third party hardware.
References/resources:
Manual (requirements, installation, use, etc): http://music-server.net/help/
Forum: https://www.loxforum.com/forum/projektforen/musicserver4lox
Installation
Briefly, steps for installation are:
Download Ubuntu Server LTS from https://www.ubuntu.com/download/server and install.
Download MusicServer4Lox installation script from http://music-server.net/download/install.sh
Execute script (as root, 'bash install.sh')
Configure (see http://music-server.net/help/ for details).