Double check permissions you've granted in the OS. It's in the install thread.
If you are asked for accessibility permissions you must accept that (it's how MC deals with retina displays).
Not using a Retina display as my primary display and the install didn't ask for this. (Not sure where to enable it in Settings.)
If you are asked for screen recording permissions, you need to accept that for MC's user interface to operate properly, MC uses that permission to copy from part of it's window to another part of it's window. It's not used for anything else.Not worried about the UI but tried this in case it helps.
If you are asked for permission for input monitoring (capturing keys) you must accept that if you want the media keys on your keyboard to work in MC, it's not used for anything else.Not using my media keys, though enabled this in case it helps.
When the system asks for permissions for MC to do disk access (Music, Pictures, Movies, Removable drives, etc) you must accept those to be able to access your media or CD/DVD drives or mounted NAS filesystems, or external drives)Accepted. It can see my files.
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I should probably make clear. I can play my files on the computer. And I can PLAY TO any of my Sonos speakers. I can see the SONOS end points (and others) in the JRiver MC interface.
What I cannot do is access the JRiver server/service from inside the Sonos controller app. I can see a different Upnp server (eg, AssetUPNP).
So the bulk of what should be working is working. Either I am missing an option to toggle in MC, or Sonos is just too finicky. Based on past experience, I'd guess the latter. (I have contacted Sonos, but their responsiveness is not great.)