Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
If several of the new music files you downloaded to the iTunes library, are of a file type or encoding type that your Sonos app does not support playback of, then that would explain the problem.
But, what are some of the file types that your Sonos app is refusing to play? specifically, so that I may check on whether the Sonos app really doesn't play back those file types, or other problems that might be present in this situation.
Also, I advise you to make sure that the new music files you downloaded to the iTunes Library, were really downloaded to a local drive or source... they might not be accessible over the network for the Sonos app to play, under some circumstances, if they were not on a local drive on your computer.
What version of Sonos are you using?
I would advise getting the latest version of the Sonos app, if possible.
Sometimes, Sonos will not play m4a files that are 'Protected'; also these files will be playable in iTunes but not by Sonos.
Also make sure that there isn't an older version of the files in question, in the Trash folder or similar place - that can somehow incline Sonos to not play the files.
Good questions! frankly, at this point I'm not sure why they don't show up in the Sonos Music Library. Maybe there's a Sync operation you need to do? or is that the Update Music Library operation you just did?
What device is Sonos (now version 6.0) running on ?
The m4a files recently taken from CDs could be protected files, or it could be some other reason that is causing the problem. (As I said, I'm still not certain why.)
Did you buy the files from iTunes? if so, DRM Protection would keep them from being played on all but iOS devices. In iTunes you can find out whether DRM Protection is on the file or files.
If a file has a .m3p extension, it is protected.
In ubuntu or other Linux contexts, protected files generally have an .m4p extension rather than .m4a ... and evidently yours do not.
Did you buy the files from iTunes?
Do any of them have an extension with a 'p' in them?