Here are discussions of two methods that are being used by Sandisk users.
- Using Windows Media Player with subdirectories of music folders copied to the player (under Music).
https://forums.sandisk.com/t5/SanDisk-Clip-Sport-Plus/M3U-playlist-empty/m-p/364455#M332
- Batch file command for playlist creation for subdirectories of music folders copied to the player (under Music).