Things You'll Need
Instructions
Connect the SD card used with your Nintendo MP3 Player (or a new SD card) to your computer, using either an SD reader or your computer's SD slot.
Use "My Computer" or "Finder" to navigate to the root directory of your SD card, which will show up as "SD Card" or "No Name."
Create a directory on the root directory of your SD card called "MP3." If you wish to sort your MP3 files into categories or play-lists, create multiple folders: "Pop," "Game Music" etc.
Copy the MP3 files you wish to play on your Game Boy Advance SP into the folder(s) on your SD card, then eject and remove the card.
Insert your SD card into the SD slot of the Nintendo MP3 Player. Insert the Nintendo MP3 Player into the cartridge slot of your Game Boy Advance SP.
Turn on your GBA SP to launch the MP3 player. Use the D-pad to highlight the directory containing the files you wish to listen to and press "A" to enter the directory. Navigate between files with the D-pad and start playback with the "A" button.
Stop playback with the "B" button or by turning off the GBA SP.