Well thats proves sounder ok.
Check on panel pcb. There will be a two or three fuses. From memory, think 1 for battery (1.6A), 1 for aux 12v (1A), and 1 for bell/strobe (1A).
With all power off (mains & battery - sounder goes), gently prise out each fuse in turn and pinch together the lugs of the fuse holder (1pair per fuse) and then put fuse back in (dont mix up fuses). Re-connect battery, shut lid then re-appy mains. Trigger alarm.
If no joy, I reckon you need a new panel.