New Era 300 will not play local library.

 Sent a diagnostics concerning the issue where the Era 300 will not play local library.

Power down ALL Sonos, reboot router and controller, power up your Sonos.

If it works and keeps working you should be good. If it works for a while then quits, you may want to set static/reserved IP Addresses for all Sonos from the router's DHCP Settings page. Follow with another network-wide refresh to lock in the new addresses.

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You may need to reindex the library to force a copy to the new device.

How is your local path formatted?

I had this exact issue with newly added Ultra and Era 300's failing to play local sources when all other, pre-existing, speakers were fine.

My local paths used a named nas device, //NAS/Library.

Changing the name element to the nas IP fixed the issue, //111.111.111.111/Library.

Edit: I should add this was at the suggestion of Sonos 2nd line support.

Remember, Sonos Support doesn’t live in these forums, only Sonos Forum moderators. You might want to call in to have someone look at your diagnostic. 

 Scan for new content worked from Windows 10 PC.  That did not fix Era 300 not playing my music library.  Tried Scan again from the iPhone.  It failed.  Sent diagnostics.

 Swiped out app.  Scan worked after that.  Oddly an Xfinity connection test showed 300 Sleeping.  A follow up Xfinity test showed strong connection.  That’s why I’m scanning library while the 300 is playing streamed music.

 The 300 still refuses to play my Music Library.  Sending it back.

Since you seem to be using a named nas in your path, did you read my post about library path formatting? You are having exactly the same symptoms that I had...

User117655

 I may be wrong here, but it seems to me if there was an issue with using the NAS network name then the library would never scan and there would be problems with playback of the NAS library.  Right now only the new Era 300 refuses to play the NAS library and the scan works after swiping out the app.

 I will try the IP Address and let you know what happens.

That is what I thought but like I say exactly the same issue you have with new speakers (ultra/era/sub surround set) on an existing system failing but with pre-existing speakers remaining 100% functional (and scanning OK from desktop app)... Changing to IP and rescanning solved it for me 🤷🏼‍♂️

(Should add I use a fixed IP address for the nas)