A while back, Sonos reorganized their FAQ library, so some of the links might be stale. Here is probably the best place to start:

https://support.sonos.com/s/article/257?

You'll also want to make sure you don’t have any firewall or virus protection apps running that might block the speakers from being able to reach your library.

@neuropathy if you are using the macOS Catalina version then you need to consider the security and folder sharing options - bear also in mind that the folder structure in the Music App is different then iTunes.

I accidentally selected “Best Answer,” however, this issue is not resolved, although there has been some progress.

By changing the folder sharing options, as el rubio suggested, I was able to get past the error in the Sonos controller, which has allowed me to add the folder to the Sonos Music Library, however, the music in the folder does not appear in the Music Library, despite updating the Music Library multiple times.

Before I had asked this question, I was able to add the /home/user/Music/ location, but not another directory that I use to store music, which is in the /home/user/Downloads/ location.

In order to add the /home/user/Downloads/ location on my Mac, I entered System Preferences > Sharing, then selected File Sharing (which must have checkbox beside it checked ) and added the /home/user/Downloads/Music/ directory to the list of shared folders. I did not alter the permissions.

However, the music in /home/user/Downloads/Music/ does not appear in the Music Library, even after updating the Music Library multiple times. I checked that the update is working by copying a test album in /home/user/Downloads/Music/ to the /home/user/Music/ directory, which worked properly after updating the Music Library. I removed the files, updated the library and the files disappeared. Currently I only have one album in the /Downloads/Music/ folder that I’m trying to add to the library.

I checked the folder settings of each directory by using Command + I (get info) and found that both folders have a user called SonosDMS with “Custom” permissions under Sharing & Permissions. The only difference I can find that is that the /Music/ folder isn’t checked as a “Shared Folder” while the /Downloads/Music/ folder is checked as a Shared Folder, however, this is the directory that isn’t being indexed properly somehow. The only other difference is that /Downloads/Music/ has an additional user called “staff” that is given “Read Only” permission, which /Music/ doesn't have this user at all.

Does anyone have any ideas what needs to be done to fully resolve this issue? I’m using a Mac running Catalina (macOS 10.15.3) and a Sonos Play 1. 

Can someone please remove the Best Answer from the response above - I accidentally clicked on this and have no way of undoing it.

Hello @neuropathy. Welcome to the Community. I understand that you are having some issues with adding music from a particular music folder. It seems like we may have the wrong folder added.When you tap on “Update Music library now”. Sonos will index the folders that were added to it before. In your case, it sounds like it is updating folder, which has no music on it. I recommend removing that folder, to prevent any confusion. 

Now, when it comes to permissions for the folder that does contain music. Please change the permissions to read and write for all users, including the staff. Once that is complete, please attempt to add your music folder. What happens?

I would also recommend giving our support line a call when you have some time to spare, as we can troubleshoot this issue better with you in live time. Our phone team have the tools to remote into your computer, and walk you through some additional steps 

Thanks everyone, each response got me a bit closer to the solution. 

What needed to be done, other than the work I’ve detailed above, is an allowance made on the Mac in System Preferences > Security & Privacy > Full Disk Access - Sonos and SonosLibraryServer needed to be added. Also, check Files and Folders and make sure Sonos and SonosLibraryServer are in there. I believe that I only needed to add the Sonos application (from the Applications list on the Mac) to Full Disk Access and that SonosLibraryServer was already there. I think Sonos needs to update the system for the latest macOS, so it works smoothly. 

I am now able to index music from the folders of my choosing on my Mac and play them through the Sonos Play 1.

Although one of the responses above is marked as Best Answer, these messages were only general responses about permissions and other users would need to read my messages to get a clear understanding of how this is actually accomplished. 

Thanks again

I had a similar problem on my Mac. If you delete the Sonos file in your Shared folder (Macintosh HD / Users / Shared ) , then re-open your Sonos app it will create a new file with the correct sharing permissions. That worked for me, anyway. Good luck.