Some of the songs show up sometimes and at other times they won't. It almost seems random. I think (though it could be coincidence) that the songs I add to the card first show up on the music player app. Doesn't matter which songs, the app just shows the songs I add first.
It may be a problem with my card. When I added 2,400 songs, 1,470 came up on the app, so I thought the app would only read 1,470 songs. Then I put my card in with only about 1,400 songs and the app only read 1,100 songs. It's weird because I know the songs can be read by the app but if I put too many songs on my card some will go unread even if I've been able to add them before.