This is mentioned in this thread: "Music Library Settings" missing | Sonos Community

I have successfully been using my NAS for years but alas with the new app it has vanished.   When I found time I have re installed it with the correct network path, and on these days it has worked perfectly.  However when I return to the app using iOS, Microsoft or Android the music library function is again missing..  my Nas is still there and working perfectly with my laptop & DLNA TVs- so what is the problem with Sonos?  Set to SMBv4.

Same here, power up my laptop, re add the library, day or two later it’s gone again.

I received this message from Sonos Support on 15th May:

Hello Peter,

Thank you for sharing your feedback about the new Sonos app. We understand how crucial it is for you to play music from your local library and search for it via the Sonos app.

We recognize your disappointment that these features are not currently available. Rest assured, your feedback has been heard loud and clear. We are committed to providing the best possible experience for our users, and we are working diligently to bring back these important features in the coming weeks.

We plan to reintroduce the ability to play and search music from your local library next month. For more information on upcoming updates, please refer to this article. In the meantime, you can use AirPlay (for iOS devices) or the Sonos desktop controller as temporary solutions to play your local music.

Thank you for your engagement and for being a valued member of the Sonos community.

Best regards,

Sonos Customer Experience Team

June has come and gone and still no sign of the on the alleged update to reintroduce the local library functionality