Okay, discovered this entirely by accident but just succeeded in getting this working. Those that are having trouble, try this…
- Make sure your SONOS device is connected and added to your Things list (I know, obviously!)
- Create your playlist in the native app and begin playing the song
- Go to the SONOS device in your Things list and open it. Press Play/Pause/Refresh until the song being played is listed
- Go through the motions of adding the Mood Music app again. Hopefully you should see the current song in the list to be able to add it.
It does add all the songs that were in the list even though it only lists one. Emptying the list from the native SONOS app seems to have no negative effect on the playlist you added to the smart app.