I’m on a Mac running Monterey and still using Sonos 1 speakers, which I’ve had for years. Yesterday I upgraded the app (Sonos1 - not Sonos 2) to 11.9 and after adding some music to my library, Sonos would not index my music folder (on a external USB).
I spend 20+ minutes on chat with “tech support” who could not find an issue, even after my sending them a diagnostic report.
Long story short - if your having this issue, go into your System preferences, select the Full disk access folder in the left column, scroll thru the list and make sure SonosLibraryServer - is turned on.
For some reason it appears to get installed, but not turned on. Even after giving Sonos system permissions whenever I was prompted, that particular portion of the app still needed to be checked on manually. Once I did that, the issue was solved.
Hope this helps anyone that might be having a similar music indexing issue.