Yes, I have the same problem.  Sonos = no joy.  So much money spent with so little support.  They do (after a good wait) answer the phone, but the first line support agents are clueless.  It’s an SMB version issue of which Sonos is well aware.  Try to get the issue escalated, and I hope you have better luck than I.  All my ripped CDs and almost 400 GB of music gone - Poof!  

Somos, please fix this as soon as possible, we’ve been waiting quite a long time now. At least Gus is the option to choose SMB1

It was supposed to be solved by mid June

new date?

Have you not changed to SMBv2 to get it to work? I don’t use libraries but access to libraries is still there. There are about a dozen threads on how to make it work.

I finally solved this (at least for me) after having 3 failed and lengthy support calls to Sonos and following the steps that other users have posted.  I’m running Sonos 14.5 on a Mac Mini with Apple Silicon.  I’m no expert, but here is what I did:

1.  Settings -> General -> Sharing.  Disable (yes - disable) File Sharing and Media Sharing
2.  Settings -> Wifi -> click the “Details” button to get your IP address, write it down (e.g., 192.168.xxx.xxx)
3.  Settings -> Full Disk Access -> and uncheck anything to do with Sonos (for me it was “smbd”, “Sonos” and “Sonos Library Server”).
4. Using the IP address you wrote down, write down the path to your iTunes Media library with pen and paper.  I have my library stored on an Samsung external drive attached to the Mac Mini.  I used this path \\:192.xxx.xx.xxx\SamsungEXT\iTunes Media
5. Note:  your iTunes Folder Tree on the drive where your library is stored should have “iTunes Media” as the enclosing folder.  Your “Music” Folder should be a folder under the “iTunes Media” enclosing folder.  Under the iTunes Media enclosing folder will be other iTunes files/folders such as the *.itl and .xml files as well as “Automatically add . . . “, etc.
6. Reboot or shutdown the Mac and restart.  I did a cold boot (I shutdown completely and restarted). 
7. Settings -> General -> Sharing -> enable Sharing.  Enable Full Disk Access for all users.  Click the info icon and using the + or - buttons, add the following folders:  the name of the external drive where your Music library resides (this should be visible to you in Finder), along with the “Music” and the “iTunes Media” folders on that drive.  Make sure that all users and admins have Read and Write permissions for each folder and user.  Also, if “Sonos DMS” does not show up as a User, try adding it manually.  Under options for each users (including Sonos DMS), make sure “Share Files and Folders using SMB” is on.  
8. Make sure both File Sharing and Media Sharing are on (although I suspect that Media Sharing is not necessary, but what do I know?) 
9. Settings - Privacy & Security - Full Disk Access, make sure “smbd”, “Sonos” and “Sonos Library Server” have full disk access.  
10. Settings -> Privacy & Security -> Files and Folders -> Music, smbd, Sonos and Sonos Library Server should be listed.  If they are not, try adding them manually. 
11. Settings -> Privacy & Security -> Media & Apple Music.  Ensure Sonos is there with permission.  If not, add Sonos manually.
12. Open the Sonos app and click on Manage Music Library.   Add the path you wrote down using the NAS option, NOT the external drive option.  
13. Things should start percolating - if you have a large library, it will take a good while for indexing to complete. 
14. This took 4 days of trying everything under the sun and the above steps are what finally worked for me.  None of the first level Sonos techs were able to solve the problem.  I’m certainly no expert, but for me, the key was the disabling of Sharing permissions, rebooting and then re-enabling Sharing permissions that eventually led to everything working.  It’s been 5 days now and it all still works!  Of course, Sonos will probably release the updated App tomorrow that fixes this horrible bug - thanks so much, Sonos.  
15. For those of you who fear that you’ve lost your Music Library, I suspect that it has just been misplaced and well hidden by the new and “improved” (haha) app.  If you have any doubt, navigate to the folder where your music is located, and if it’s there, you haven’t lost it.  I guess now would be a good time to copy the library to a backup drive.  Good luck everyone and I’m sorry if this doesn’t work for you or if I’ve missed a step or further confused anyone.  I’m no techie, just an average Joe who is persistent and sometimes successful.