Tuxemon's music is stored in the Ogg Vorbis format. This is similar to the MP3 format, only it is completely free. All music shipped with the game can be found in the music folder in `resources/music`.
Using your music in Tuxemon
You can use your music in Tuxemon by placing it in the `resources/music` directory and creating an action with the action type "play_music" on the map of your choice using Tiled. Then set the "parameters" of the action to your music file name.