Apple Music App (Golden Gate MacOS, Mac Studio) won't play parts of a 20 year, 10,000 song library.

A random mix across CDs, purchases and years usually means those tracks never came over as files. On the iPhone they stream from your cloud library; on the new Mac the entry points at a file that isn't there. Right-click one that won't play and choose Show in Finder: if Music can't find it, that's the cause, and copying the old Music folder over from the previous Mac fixes it. For the older store purchases also do Account > Authorizations > Authorize This Computer, since pre-2009 iTunes songs are protected and won't play on a Mac that isn't authorized.

Katalog https://musickatalog.com runs alongside the Music app and lists every track whose file is missing, then relinks or copies it back in (I built it).

That .m4p clue is the key one. The iTunes Store has sold DRM-free .m4a since 2009, and Apple Music launched at the end of June 2015, so .m4p files from then on are almost always Apple Music subscription downloads rather than purchases. Those only play while you're signed into Music with that subscription and Sync Library turned on; a Mac-to-Mac transfer copies the files but not that link. Anything you actually bought is in Account > Purchased and re-downloads as plain .m4a, so delete the .m4p copy and download it again from there.

Katalog *** runs alongside the Music app and has a File Type column, so sorting by it lists every .m4p in the library at once (I made it).

[Edited by Moderator]

Of course I appreciate an attempt at a response-

The files are there for the ones that will not play. For example, I have a folder that corresponds to a long ago (June of 2015) partial purchase of an album. One song I purchased plays, the other two don't. All three files are stored in the same folder on the computer.

And as part of trying to fix things, I've deauthorized and reauthorized multiple times. No change. And the problem shows up with files purchased long after 2009 (the files above).

But I think I have found what may be wrong. The files that won't play are .m4p, and the ones that will are .m4a. How the same album had a mix of .m4a and .m4p, I do not know, but that's what it shows.

So clearly something is wrong with my DRM protected files. Why they worked on the computer I just replaced and not on the new Mac Studio, when I did a straight apple transfer is another mystery.