I have S4 and same SD card as above. Exact same issue. Can not get Samsung music player to see it or import library. I can use MyFiles and select a file and play it even, just not through Samsung app. I even set the default path to my SD card and music folder. It does not see it. I have reboot the phone and used Rescan SD app too. I have mounted and unmounted. No problems, just won't be picked up by Samsung music.
If I copy the files from the SD card to the Music folder on built-in device storage, they pop up just fine.