First, the player needs its own folder to store mp3. For example, the original Apple mp3 player needs a special folder to store mp3, and mp3 will only be found and played by the player if it exists. It is recommended that you format the SD card and put it in the player (some players can create this folder when inserting the SD card). If not, try to create a folder yourself.
Second, the code rate of mp3 is incorrect. Because some players can't read and play high-quality music of 192kbps. Download the music file with the code rate of 128kbps by converter or download.
Third, the SD card reading part has poor contact. (Usually this is not the case for new players or players who haven't used it for a long time. This kind of situation is rare unless there is something wrong with the quality of the machine itself. Erase the SD card pins (yellow pins on the SD card).
If the problem is solved, please give your opinion. Thank you.