I am not sure if this is an eOS only issue, or if it has to do with the encoding of the music files. I converted my music from FLAC to .ogg in order to save them to my phone. I saved them in the "Music" directory and the Music app scanned my library. I have a lot of music, so I didn't realize immediately that not all of my music was showing up in the app. They play fine if I go to the directory and open the files from there, but the app doesn't seem to recognize them or show them as available.