Dave Liquorice expressed precisely :
Does anyone know of a sounder suitable for mains use that won't deafen
people within 10 yards. Ideally all I want is quiet beep 15 sec pause
beep 15s and so on.

Mains powered ones tend to be robust and er, well, loud.

Why not find what you need in a low voltage version and feed it from a
small wall wart? 12v output wall wart connected to a vehicle reversing
alarm perhaps.

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Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk