This is what's probably supposed to happen! If you cut the power supply to the siren the internal battery in the siren should sound the siren - it's a fail safe to stop the n'er do wells cutting the wire to the siren to silence it. If the siren doesn't sound when you disconnect the power and battery then the battery in the siren is kaput. If the siren sounds when you cut the mains power to the alarm, then the main battery is kaput.
What you should do is: enter the engineer's code, remove the cover from the siren and disconnect the internal battery, then you can remove the cover from the main unit, turn the power off, and replace the battery without the main alarm sounding. If you haven't got the engineer's code then you can open the main case which will set off the tamper alarm, enter your normal silence code, then remove the battery from the siren, then change the main battery.
We never set ours, so it got the heave ho when I fitted a new boiler and the flue went right through where the siren was. I worked out the above by trial and error and the internet. Then I found the installation instructions about 3 months later :-))
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Changing a burglar-alarm battery? new
Changing a burglar-alarm battery? new