Do you have both Play:5s wired to your router? Have you tried just having the left speaker wired to see if that makes a difference?

@GuitarSuperstar I tried having both wired and having only one wired, and it made no difference compared to connecting them wirelessly. 

When you have just the left speaker wired, in the Sonos app, if you go to System > About My System, do both speakers show “WM: 0”?

I’ll have to check that in a couple days when I have the chance to fiddle with this again. I’ll let you know ASAP

@GuitarSuperstar One of my Play 5s connects to ethernet just fine, but the other one has the ethernet “link” light on constantly, even when no cable is connected. No matter what I do, I can’t get that speaker to connect to ethernet, but the link light is always on. 

When you have just the left speaker wired, in the Sonos app, if you go to System > About My System, do both speakers show “WM: 0”?

When one speaker is plugged in, only one speaker shows “WM: 0”, the other one shows “WM: 1”

Have you tried resetting the Play:5 that is not connected by ethernet?

Follow these steps:

1. Unplug the power cord from the electrical wall outlet.

2. Press and hold the Play/Pause button while simultaneously plugging the power cord back into the electrical wall outlet. Continue holding the Play/Pause button until the light begins flashing amber and white.

3. Once reset, the light will flash green indicating the product has been reset and can be added back to your Sonos household.

Yes, I have. Even when an ethernet cable is plugged in, it does not show up as WM:0

Sounds like you might have a defective unit. I would contact Sonos support.  https://support.sonos.com/s/?language=en_US