Hmm, ok. Was hoping for something more direct as I saw in the docs
notificationsound | String | Play a notification sound by a given URI
The texts for the notification service, reason and message appear on the device display (when available) and the SoundTouch application screen. Upon completion of the notification, the speaker volume returns to its original value. If not present, the notification will play at the existing volume level.
So I was hoping to be able to ring a bell mp3 with that parameter but could not get that working.