Stop sounder on Accenta G3 when power off?

When there is a power cut, the siren sounds on my alarm (not full blast) I was told that the backup battery was dead, but if that is the case why does the sounder go off. I entered the engineer code to check the battery but as soon as I switched the power off the alarm sounded. Can you advise me please. My meter is going to be changed shortly and it's going to take about an hour, so that means my alarm is going to annoy the neighbours.

Den
September 2008