Panasonic KX-TGFA30 ringer volume too low

I'm trying to fix my dad's phone handsets. They're for his business and he carries one of them on him at all times while the other two charge so that he can always have one available. He's very hard of hearing these days and the ringers in the handsets has gotten too low for him to hear.

The way he describes it to me is that everything was fine and then suddenly one of the handset ringers started getting quieter and quieter until it's barely audible (even to me with normal hearing) so he started using another one. It also started getting quieter and quieter. Finally the third one did the same. He says that it all happened over the course of a couple of weeks which seems oddly coincidental to me. These phones are only about a year old and I've already tried replacing the batteries and I've checked that the ringer volume is still maxed.

One more thing that I've noticed while messing about with the phones is that the menu selection beeps sound normal volume and in fact sound slightly louder than the ringer does. They both come out of the same speaker on the phone (the one on the back of the handset, not the earpiece). This leads me to think it's some kind of setting or firmware issue rather than a hardware issue.

Any ideas? I would assume that the ringer speaker itself had just gone bad if not for the fact that all three handsets had the same failure within days/weeks of each other after a year of use.