[0] - https://audiocast.io/
Airplay 2. I have it hooked via optical to a soundbar.
Anything involving Sonos with wires increases support cost. Simply involving wires produces new ways for people to screw things up (as if supporting their wireless-only customers wasn't expensive enough).
And those who are using wires but not screwing things up gain the ability to expand and improve their audio systems in their own ways, without paying the Sonos tax to do so. They can buy/upgrade whatever speakers they want from any manufacturer in the world, while keeping the same Sonos Amp. That represents lost opportunity, so the amp gets taxed harder.
(The tech inside the Sonos Amp isn't particularly novel. It's just a small Linux box with a trick proprietary software stack and some audio hardware.)