From GameBrew - a wiki dedicated to Video Game Homebrew.
3ds-vgmstream is a port of vgmstream. Initially created by hcs, it is a library for playing a wide variety of streamed audio formats used in video games, comes with source for Winamp, xmplay, Audacious, and foobar2000 plugins, and a command line decoder that outputs PCM .wav.
This port is based on vgstream with a simple player (for now) utilizing the library.
Extract the archive from releases to your SD card.
Place any tested file formats in the music directory on your SD card.
When the application is opened you are presented with a list of the files it found in the music folder.
Please see this list.
Theoretically playable formats
A list of supported extensions is available here.
Note that while vgmstream supports those formats, not all will load correctly on the 3DS (some tweaks will need to be made to have it work on the 3DS).
Developer asked if you find that a file format doesn't work then please comment the file format in this issue.
Up/Down - Scroll through list
A - Play selected file
B - Stop
Start - Exit application