I have the Pfaff QE 4.0 and my alarm is quite loud. When the bobbin is low the alarm sounds and the machine stops sewing. If I have the machine set up to do FMQ with sensormatic and then turn the machine off, when I turn it on again the alarm sounds loudly to tell me it is in sensormatic mode. I would pursue the matter is it were me becasue the alarm should be usable.
I also have a pfaff 7530 and much prefer the way it tells me my bobbin is low. A red light starts to blink but the machine keeps sewing so I can run the bobbin right empty.