Bill Wright explained :
If you can find the actual sounder you could simply snip one of the wires (or
fit a switch), then tape up the end so it doesn't short on anything. The
sounder is likely to be a little round thing somewhere under a panel at the
back. Get someone to find it by listening.

I've looked through the user manual and can't find anything. The maintenance
manual I can't find on line.

A blob of bluetac stuck over the sounder's hole, would almost silence
it.