In external Mode Dozzzer reduces the system volume without playing anything itself. You can you any other app to play music, podcasts or audiobooks. It uses multiple techniques to stop playback in the other app, which works best with Apple’s music or podcast apps, or if Dozzzer is running in foreground.
Music Mode plays music from your music library right inside Dozzzer. Dozzzer can regulate the music volume directly without changing the system volume. However this works only for DRM-free, downloaded tracks and not for Apple Music streaming, since Dozzzer cannot decrypt DRM protected music.
Since Dozzzer cannot play DRM protected music, please use the external mode and Apple’s music app to play Apple Music tracks. This works even when Dozzzer runs in the background.