JEplayer can only play DRM-free files in the "MP3" format. It works best with files that are stereo, a bitrate of 128kbit/s or lower, and with a sample rate of 44.1kHz. If you have other formats or bit rates, you can use free software like Audacity to do the conversion.
Insert the Micro SD card into the USB reader, then insert the reader into your computer.
Next, create a folder in the Micro SD card with the name of your playlist or the title of the album.
Then, copy MP3 files into the folder.
If you want to ensure that they are played in a particular order, name them "01 - First Track Name.mp3", "02 - Next Track Name.mp3" and so on.
For each additional album or playlist, create a fresh folder in the Micro SD card.
JEplayer only looks at folders in the top level, so remember not to create a folder inside of another folder!
After you've added a bunch of albums, the top level of your Micro SD card might look like this.
Here's what a typical folder full of music should look like in the file browser. Remember, this needs to be a folder inside the Micro SD card.
Need some music? We can heartily recommend Bartlebeats - Frequency, the official soldering soundtrack of Adafruit.