And you’ve rebooted both the Amp and the Mac?

Yes. 

Has it worked okay since you installed the new WiFi mesh?

I don’t think I’ve used it with my Mac since I installed a new Wi-Fi. I have used it with my iPhone however. And it still works with that.

Could the iPhone and Mac have connected to different mesh points? Or is the Mac wired?

Some Sonos details would be useful. Is the Sonos connected via the WiFi, or is there a wired Sonos component? Which WiFi mesh is it? 

No, there’s only one mesh network. The iMac is connected via ethernet to the network, though wifi is on for some macOS features. 

The Sonos amp is connected via wifi. It’s a Netgear Orbi. I had an older Orbi for five years, and almost never had problems. 

I mucked around in the Mac’s network settings, and I was able to play for a while. Then I went to have lunch, when I came back, it failed again. I’m perplexed, because it’s just this Mac; my MacBook Air and my iPhone can stream to it with no problems. That suggests that the problem has something to do with Ethernet, but AirPlay works over Ethernet; I’ve used it like that in the past. 

There’s a slim possibility that some necessary traffic is not getting between the wired network and the Orbi WiFi. It could however be something in the way the Mac connects to the Ethernet.

It works fine via wifi; there are macOS services as part of Handoff and Continuity features that use wifi, and they work fine. 

It looks like if I disable ethernet in the network preferences, then it works. But it always worked before, so I’m at a loss. 

You could wire the Amp and see whether it works Mac→Ethernet→Amp.

If it doesn’t then the problem is probably in the Mac’s Ethernet interface.

If it does work, then there’s likely some issue in the network when it’s Mac→Ethernet→Orbi→WiFi→Amp.

One of the reasons I got the Sonos Amp originally was to have one less cable. The Mac’s ethernet interface works fine; I have fiber, and I get 1 Gbps when I do a speed test, which I wouldn’t get over wifi. There’s something going on that’s blocking AirPlay over ethernet, and, again, it always worked before. 

I was taking about temporarily wiring the Amp, in order to try and isolate the fault. 

Yes, that works. I have a breakout box connected to the Mac that has an ethernet port, so Orbi > Mac (ethernet) > Sonos (ethernet) works fine. It must be some odd setting in the Orbi that is preventing it.