I can not hear the notifications or hear the spoken message when I receive. By the time I get the alert facebook for example, the volume is down automatically to almost 0, and when the message finished talking the volume returns to normal.
I'm not listening to music or using applications ringing sound when the warning comes. I searched settings but found nothing.
It happens to all notifications, all that is notified when the volume drops to the minimum.
Tested in two mobile Sony Xperia ZQ (Brazil) and Xperia Z1.