Aha! These files are no mp3 but wma files. There have been reports of some trouble to read meta tags from wma files.
Were these meta tags already in the files when you added them into Plex?
If not, try the Plex Dance with the whole album.
(with music, you can omit step 4 of the dance)
If you still have the original CD, I recommend you to rip them again but this time to use the mp3 or the aac file format. Because these are much better supported.
