The music player would only access the mp3 files stored in the default storage location on the internal sdcard. My own music files were located in a folder on the external sdcard, and initially were inaccessible from the music player. I got round the problem as follows>
1. Cut and paste the external music files to the location on the internal sdcard where the music player currently sees the files. In my case I did this via a windows PC with the phone connected to the PC, but its probably not necessary - could just use the 'move' and 'paste here' functionality within the My Files app.
2. Verify that the music player can see/play all the newly added files on the internal sdcard.
3. This is the important bit. Use the My Files app to 'move' and 'paste' all those files just added to the internal sdcard back to the external sdcard to a location of your choice, the folder name is irrelevant.
On opening the music player it should now see the files located both in the internal and external sdcard locations (at least this worked for me). I've processed all my music files in this way and my music player happily plays all my mp3 files which I exclusively keep on the external sdcard.