Model numbers help, Are these PS-1 or PS-2. If you look at your manual there are instructions for a conventional reset. For PS-2, one Whistle and 5 Bells in rapid succession.
Other issue can be a bad speaker, especially for PS-2 5V.
Other issue can be corroded terminal for battery, based on old battery left in too long and gassing out.
So both can be simple fixes. G