Yesterday pulseaudio got installed again on my system. I have noticed that basically, when I press the keys for volume+ and volume- on my keyboard, in that screen only the virtualbox and amarok sound levels change, while the notification remains stuck at maximum (it was at maximum when i opened the mixer).
If I lower it manually, it remains lowered, doesn't move with the others when I change the volume. I suppose this is the issue. I'll now remove PA again, since I don't trust that volume bar to stay where I put it, the moment that a notification will actually come.