Plex is a nightmare for music. The way it does it is actually broken, and works only if you follow certain rules.
Firstly, it does not do mp3 id tags the way you would expect, bear this in mind.
1. How the albums / music appear on the system is a function of how you have the files saved on the disk, in folders etc.
i.e. if you want 2 tracks to appearon the same album, you must put them in the same folder, the id3 tag is irrelevant.
it must follow artist/album/01 track.mp3 structure
music brains can sort all this out
2. if you add more tracks to a folder after it's already 'ingested' into the plex, plex will see this as a new thing and it will appear 'split' in the listings. Solution - remove the whole album/folder, and then re-add as a single item. (you may also have to do 'the plex dance')
3. the id tags are used to display the artist/ track name etc so you use the folders structure to make them appear the way you want, and the tags to have the labelled/named the way you want. If you mess about with tags and metadata after ingesting to plex, to get it to update you may hev to do the plex dance again.
4. Even if you tick - use local metadata plex still sometimes has difficulties. It really really wants to and works best when you have plex get all the metadata using it's own engines. Sometimes you have to correct matches etc. There documentation on this on plex website.
5. The plex app interface no longer does music, so to play back you should use Plexamp on your phone etc.
6. Why oh why oh why couldn't they just do a simple tag based mp3/music player - i haven't found any good solutions yet.
edit i meant to say that for compilations and various artists, if i remember the way to do it is to have the album artist as 'Various Artists' for each track, and the album tag should be the same. and all the files for the album should be in the same directory on your disk