Not sure if this will work for your setup, but have you looked at snapcast (github.com/badaix/snapcast)? It’s a two part solution. You install Snapserver on the mopidy/MPD enabled box you want to have audio synced to your other boxes running snapclient. As long as you specify what stream snapserver uses, you should be ok. Depending on what program you are integrating this with, you can also run multiple instances of your music streaming app (ex mopidy) so that one instance streams to Snapserver while the other goes through your existing sound system.
I haven’t tried this with Volumio yet, as one thing I was trying to do is run the server on the same machine as one of my individual system, but Volumio only allows one instance to run. So, for fun and to experiment I tried using Mopidy and have gotten the multiroom audio working with Spotify no problem; haven’t tried running multiple instances yet though.
You can read a bit more on it here too. home-assistant.io/blog/2016/02/ … -snapcast/