iOS
iOS users can integrate Apple Music (or their own music from the Music app), podcasts and audio books into their timers.
Android
Android users need to upload music directly to their music directory of the device. Music should not contain Digital Rights Management (DRM) or it will not play in Seconds.
Details of how to load music on to your device with a USB cable can be found here:
Spotify, and other streaming services.
Spotify is probably the most popular music platform available. However, we do not support it within the app as they have prohibitive terms & conditions, and a poor API that would actually require switching to and from the Spotify app each time you change an interval.
You can only play music from streaming music services such as Spotify, Google Music etc in the background while your timer is running but these services cannot be integrated into the timers.
iOS users should make sure that the music is playing before they enter the timer screen to ensure the app is configured correctly for external music playback.