Music Library Compilations not playable in new app

That is so often the problem. The MP3 specification permits metadata that describes things like Composer, Lyricist, Songwriter, (even Librettist) but not every tag editing software makes use of these or gives you access to them. And then, of course, if you are able to edit them, there's no guarantee that MP3 player software will read all that data.

It's a long hard road, and I can well understand why people accept compromises for the sake of continuity. iTunes is not perfect in my view but it is very consistent and widely used as a standard for tagging and playing.

Mine now has compilations that show as a single album but there is still nothing in them. I have others that show as multiple albums but do contain fragments of the album. 

I was wrong in how I expressed things in my previous post. The specification that defines the use and purpose of metadata is of course ID3, not MP3. MP3 only refers to the file audio content, not the additional descriptive metadata. Apologies. When we're all suffering problems imposed by SONOS it doesn't help to get the details wrong.

  I think that the key is that all music files belonging to a single album should have the same tag value for Album and the same tag value for Album Artist. They should then have different values as appropriate for the tag Artist.

What are you using to write these tag values to your files? In theory, if SONOS doesn't change how they read this data, the writing should be a once-only task.

My, very recent, experience with v16.4 SONOS is that it now seems to interpret the hierarchy and relationship between these tags correctly as I wish.

Good luck.

These albums are just as they were copied into iTunes. I didn't change anything. They worked just fine in two homes with two different generations of Sonos equipment until July.

As far as I know the issue should be resolved for many (not all) users, if you are on latest 16.4.2 firmware AND use „album artist“ with correct id3 tag settings. 
There seem some users to be left still running into the issue. I don’t know about the reason…

So that might be the point why Sonos still isn’t talking about the latest update as a final fix. 

Going to album artists from Itunes album restored the content. But more of the compilations (some of them aren’t really compilations) now appear as multiple albums, each with some of the tracks that should be one album. For instance, my several versions of Beethoven’s 9th symphony now each appear as 4 separate albums, each with one movement. Dinah Washington’s 20th Century Masters collection has all but one song together. The one that is on a separate “album” was recorded with Brook Benton at the same recording session as one that is grouped with the rest of the songs. Frailty, thy name is Sonos.

 Not sure why your Beethoven 9th albums have only one movement, but I think the fact there are 4 separate albums is a good thing.  The way I would tag the different Beethoven 9th symphonies is…. Artist=Beethoven - Album Artist=Beethoven - Album=Symphony No. 9. Leonard Bernstein.  Obviously the other 3 would have different conductors.

 This is an example of the results:

@MoPac - Aaarrrgghh! No, no, no, no! 😒

Beethoven was never a recording artist! This outrageous error can not be permitted!
Re-edit all your tags immediately.