Dear all,
I am afraid I cannot test it either. I try to tell you my knowledge about those notifications. The notificationVolume configuration is an optional value. I suggest NOT to set it in your thing handler configuration because at the time you set it, it will be used every time you try to play/say something. Currently there is no chance to change it during runtime. When the notificationVolume is empty the current volume will be used unless you pass a volume via the say or playSound actions in a rule.