Logitech media server, every day of the week. Despite its name, it is open source and in the hands of the community. Then, with cdrummond's Material Skin and LMS Android Wrapper, it's next level good. I can't ever imagine going back

Edit: if hosting the server on a RPI, check out piCorePlayer. It can host your server and also serve as a player simultaneously

I can recommend mopidy and snapcast.
This will allow for a Sonos like setup you are looking for.

There's moode as an alternative to Volumio and only really supported on the Pi.

You can do most of this stuff manually too, but of course that's more work.

Music Player Daemon - mpd - is a small Linux utillity that does most of this.

It is a small music player runs as a service on a linux machine (RPi worked for me when testing) which can be controlled through a remote control app or desktop program.

It needs access to your music library, so look into sharing it, possibly through NFS, or set up a copy of your music library on local storage in the RPi.

I am a bit concerned about how well it would work with a shared solution, I know that some systems might lock open files preventing other clients from using them, but that is nothing I have tested.

In the past I've used Volumio, but they've made multiroom a paid feature now.

One way around it would be to run a chromecast or airplay client, as they both have inherent multizone support, just not as polished as a sonos-style setup with groupings and stereo mapping when using DIY hardware.

Roon is self-hosted but not open spurce. Incredible at playing music, beautiful UI etc. Not free but best in class imo. https://roon.app/en/

Maybe Volumio can work? It’s some time since I last tried it.