The Orbi units have Ethernet ports on the satellites as well I believe. So you could do the last mile hard wired from the orbi unit to the device.
Yes, they do, or at least most of the models do. They have a number of them that vary by wireless speed and other features.
Orbis, and a number of other mesh systems, also can do ethernet for the backhaul. With these mesh systems, strictly speaking, the "mesh" part is defined by how the various units talk to each other, present as one access point and let you manage them all together. The backhaul is how they communicate with each other vs. communicating with clients.
The better systems actually have a separate radio for this so backhaul isn't sharing bandwidth with clients. Or, the better ones can also do that backhaul communication via ethernet if you can physically run the cable. Basically, it's always better to be hardwired for network purposes, but obviously that's not always feasible or can be a big PITA. Another consideration is that consumer level WAPs can get overloaded with too many wifi clients. Hard wiring can help here.
Getting back to the Orbi, I had an RBK50 set for a bit. While it worked, it worked very well. It replaced two Apple Airport Extremes which were having coverage problems. Unfortunately, it didn't work for very long. The satellite started getting flakey after a week or two and it did it in such a way that make it a big PITA to diagnose. All the lights were saying it was connected but it wasn't accepting traffic. I'd reset it and it would work for a day or two and then back to the same thing. Resetting was also painful because I had to set it up again for ethernet backhaul which wasn't well implemented (at the time). Looking through various forums, there were many other reports of similar instability.
Netgear offered to replace the satellite but I had to send back the old one on my dime (f**k that). I returned it to Amazon and bought another Airport Extreme on shopgoodwill for $25. I've since added more wifi clients and the Airports (generally a good, solid piece of hardware) are acting like they're overloaded.
In the future, I might try the Orbi again but my most likely replacement candidate would be Ubiquiti's Amplifi HD system.
Anyway, to reiterate the previous point, with network devices, it's really about bandwidth and latency. Any consumer level network device should be fine for streaming audio even if it's FLAC or whatever. Worry about getting something that won't be a PITA.