I don’t know if any of these articles may help.

https://support.sonos.com/en-us/search?q=music+library

Are you using the Sonos Mac controller or the mobile app?

Hi,  Thanks

I’ve followed those steps in the how to guide.   I’ve asked Apple support who checked a couple of settings.  I’ve removed Sonos from my old iMac.  

On my new MacBook I’ve removed Sonos, restarted my router, installed Sonos again - just in case of some sort of clash.

I’m using the Sonos Mac controller.

I am not a Mac user, but I recall that in the past it was necessary for the “Computer name” and the “NetBios name” to be identical.  Not sure if that is still relevant but it used to trip up a few people.

Hi,   An update.   After 65 minutes on phone to Sonos support my music library is now connected to Sonos and playing.

The answer is in System Settings, Privacy and Security, Full Disk Access, and allow access for Sonos.

Further update. Sadly after playing one track, no longer connected & won’t reconnect.  V frustrating!  

Hi John, thanks for trying to help.  Where would I find the NetBios name please?

Hi.  I am not sure the term is used on Macs any more.  It is related to Windows file sharing.  I am not a Mac user, but this article may give you some clues.  Before you spend time on this though, something in the article made me think it would be worth removing the space from your computer name.  Then reboot the computer.

I’m not optimistic

https://support.apple.com/en-gb/guide/mac-help/mchlp1177/mac

Might be worth reaching out to Sonos Support directly to discuss it, as well. They’re likely more familiar with the specifics. 

Spent 65 minutes with Sonos support yesterday but …

Pleased to report I’m up & running & have music playing through Sonos.

the issue was where I was storing our music - needs to be in the Apple Music app & can’t be in a folder (like it was on previous iMac)

Also changed the name of my MacBook to remove apostrophe and spaces.

Trial & error got there in the end.

Thanks for suggestions of what to try everyone