Isolate the mains power to the alarm panel,either by removing the fuse in the fused spur or turning electric off.The sounder will continue to run at a reduced level, until the SAB in the sounder drains.
Remove the control panel cover then check all six fuses on the board with a multimeter then replace the 6Ah battery, red lead first, cover back on, mains back on, code in then reset.
If it then displays a message, get back to us and we try to help you sort it.
Ps is this one of ADT's monitored systems?
If you are unsure then get someone in.
e manno
August 2012