Elmdene Int500,
The controls has plenty of spare zones & outputs, I would not have double things up,
whilst the alarm is unset walk up to the internal speaker / internal sab.... firmly strike it with the handle of a no. 2 pozi or similar.
If if goes into tamper adjust it so it does not....
If you can't strike into tamper, do the same trick to the external bell, remember the switch in on the internal cover no on the external lid