To add background music, go to the theme options panel then Background → Audio. At the Background music setting, click Add a music track to add a track. In the track settings, give it a name, and upload your audio files. Either a .mp3 or .m4a file is required, uploading a .ogg version will increase browser support. You need to use the same chosen formats for every track you add. For example if you choose .m4a and .ogg for your first track, every other track should have both a .m4a and .ogg version.