That’s odd. When the surrounds (your Era 100s) are set to full, they certainly should be outputting full stereo music. Although I’m not familiar with the Pro versions, I can’t understand why they’d be different. If you remove them as surrounds, temporarily, and put them in their own ‘room’, and group them, do they all play music? If so, I’d set them back up as surrounds, then replicate the problem, and submit a system diagnostic within 10 minutes, and call Sonos Support to discuss it. Don’t post the resulting diagnostic number here, they get sensitive about GDPR.

There may be information included in the diagnostic that will help Sonos pinpoint the issue and help you find a solution.

When you speak directly to the Support staff, they have tools at their disposal that will allow them to give you advice specific to your network and Sonos system.

They’ll have greater knowledge about the Pro series of speakers, and likely why they aren’t working properly for you. 

Can I use Era 100 Pro as rear speakers in a Sonos home cinema system?

Yes. If the Era 100 Pros are using a wired network connection, then the home cinema primary (soundbar or Sonos Amp) must also be wired to Ethernet. If the Era 100 Pros are using their backup wireless network, then the home cinema primary can be wired or wireless.

One note I found in the FAQ on the sales page:

Can I use Era 100 Pro as rear speakers in a Sonos home theater system?

Yes. If the Era 100 Pros are using a wired network connection, then the home theater primary (soundbar or Sonos Amp) must also be wired to Ethernet. If the Era 100 Pros are using their backup wireless network, then the home theater primary can be wired or wireless.

You didn’t mention it one way or the other, but are these devices wired, or unwired? 

Hello!, yes they are currently wired.  I can group them in their own rooms and play stereo fine.  But when I switch into surround I get nothing, not even a peep.  Set it to full on music and same result.  

I will try the support route and see how it goes, it's just my luck to buy something and have it not work correctly out of the box

Not just the surrounds, is the Beam also wired? That’s the other half of the equation, as both ​@ratty and I were suggesting. But you’re certainly correct, calling in would be best. I don’t have any idea about the installed base of Era 100 Pros is, but I’d be willing to bet it’s fairly small, since it is only available through installers, and not those of us in the general public. 

The Pros would normally have a wired connection as they're PoE. If the HT primary isn't also wired the setup will fail because the latency to the satellites is too unpredictable. 

That my friend was the problem!  I added Ethernet to the beam and it's working like a charm!  Woohoo y'all are awesome thank you!

Ethernet connected to the beam was the issue!  Thanks everyone!

Correct, ethernet to the beam connected to my router was what was needed.  Some other folk also said this, thanks again