why Is my manually loaded music missing from my iTunes library?

If you've been using iTunes since 2004 your media folder is likely to be inside ~/User/iTunes and called iTunes Music, unless you've taken steps to change it, or rebuilt the library with anything from iTunes 9 onwards. The modern layout calls this folder iTunes Media. Some users have both folders inside their iTunes folder which indicates some kind of error has probably occurred during an update. I assume from what you've said so far that you've explored the various subfolders of the iTunes library for your lost tracks without success.

iTunes doesn't move media outside of the designated iTunes Media folder. It only moves media inside the media folder if the options are set to keep it organised. There is an option to change from the older iTunes layout which put artist folders in the main media folder to the newer one that puts the music files deeper in a Music subfolder to match other media kinds which each get their own subfolder. It should only do this when you choose the rearrange library function, but I've run into the odd instance where it seems part of the music was moved to the new layout but then the database not correctly saved with the new locations, leaving some files lost and the media folder in a mixture of the old and new layouts. I have a tool for repairing this kind of issue in iTunes for Windows, but not one for a Mac. If you could find the true paths to some of the missing files then we can discuss techniques for restoring an older version of your database and repairing the links if necessary. If the files aren't there at all then you need to look at whatever backups you might have to hand (which is why I asked about them) or your device. See Recover your iTunes library from your iPod or iOS device if there is anything that can be usefully recovered from it.

tt2

Don't know where to start.

So you bought a new computer.

You ripped some CDs.

You backed up your phone to iCloud. (This is probably a red herring.)

You looked on your computer and all your ripped CDs were gone.

Backing up a phone to iCloud doesn't touch music. It won't back up any of your iPhone's music to iCloud. iCloud only acts as a source of music for a phone (if you have any there). So as far as I can tell the phone backup has nothing to do with this.

You don't say if you subscribe to Apple Music or Match. If you do or do not makes a big difference in how iTunes can behave in terms of displaying music from various sources and on multiple computers. Usually when music "disappears" on a new computer is because people do not realize they were using the copy in iCloud and have not signed into iCloud again. CDs are only in iCloud if you have one of those subscriptions.

Despite great conspiracy theories abound, Apple does not reach into your computer and remove music files. Oh, people will claim that but in the end it is almost always because they use iCloud and do not understand its complexities.

I know this isn't getting to your root issue which is missing albums, but getting the distractions out of the way unless you have a subscription about which we know nothing then I am not sure where to start.

Try Empty/corrupt iTunes library after upgrade/crash.

Have you set up Time Machine or some other suitable backup software?

Apple haven't stolen any music from you.

tt2