Linux has had network audio support in various forms for a long time.

The default sound server for most distros (PulseAudio) supports TCP or RTP streaming. RTP supports multicast (and multiple multicast groups so you can break down your endpoints into multiple groups).

If you want airplay streaming, then there's shairport-sync ( https://github.com/mikebrady/shairport-sync ) that also has multi-room support; each room can map to an ALSA target (which could be a virtual PulseAudio device pointing to a network target). Reportedly (at least when I last checked a few years back) there were some issues experienced compiling with PulseAudio - alternatively you could just run shairport-sync and native ALSA on each of your targets.

Just add a few Raspberry Pis, (and maybe some dedicated DAC boards if you're bothered about sound quality) and you're good to go.