As you might have noticed, MPRIS protocol commands do not include volume control. This is broken within spotify itself, which ignores volume change requests. However, there is a possibility to control volume via pulseaudio's input sink:
As you might have noticed, MPRIS protocol commands do not include volume control. This is broken within spotify itself, which ignores volume change requests. However, there is a possibility to control volume via pulseaudio's input sink: