Make a folder on the root of the SD card called "Music." Place your files in here. They need to be a certain format, but I don't know what they are. I only know they can be MP3s.
To play them, go to "Nintendo 3DS Sound" app. Simply locate your music file and play it.
Also, each files bit rate must be between 16 kbps and 320 kbps, and the sampling frequency must be between 32kHz and 48kHz. If you are using conversion software, you should be able to adjust these values when you create the files.
To put the files on the SD card, you will need an SD card reader on your computer. If you don't have one, you can buy a USB SD card reader for relatively cheap.
1) Turn off 3DS
2) Take out SD Card
3) Insert SD Card in SD Slot if not SD adapter
4) Open the SD Card on My Computer
5) Open the DCMI Folder make a new folder called Music
6) Put Music in Music Folder on SD Card (make sure it is all .mp3 extension
7) Go safetly remove hardware and go SD