I figured out the problem. It recurred again today after editing another mp3 and uploading it to SD card. I found out that the DF player recognizes the files 0001.mp3, 0002.mp3, 0003.mp3, and 0004.mp3, if you code them as play(1), play(3), play(5), and play(7), respectively. There must be some hidden files between those numbers that are created. Hopefully it helps someone else.