On versions 3.14 and newer of the Calm app, there is now an option to enable autoplay on Music tracks.
To use this feature, simply start a track and look for the autoplay icon to the left of the rewind control in the session player. Tap this icon to switch between autoplaying all Music tracks, autoplaying the current track on loop, and disabling autoplay.
If autoplay is disabled, there will be a grey circle icon with a grey slash through it. This means the app will stop playing music at the end of the current track:
If autoplay for all tracks is enabled, there will be a green loop arrow icon. This means the app will continue to play tracks from the Music menu until you decide to stop it:
If autoplay for the current track is enabled, you will see the same green loop icon with a '1' in the center to indicate a single track. This means the app will play the current song on loop until you decide to stop it:
To switch between autoplay options, simply tap the icon until it indicates the desired setting.
Please note this feature is only available for tracks played through the Music menu at this time.