The reference Mark made was about the piezo's being faulty and making the comparison, if everything else was working ie the strobe, and no faults being reported then the piezo is responsible for the sound so that not a bad place to look if its only not making a sound.
Having looked into the manuals in more depth, the user (Master User)has the ability to change the squawk on arm/ disarm so its possible its been disabled?
If the bell unit is seen by the panel and has good or adequate signal, then I would seriously suggest that you look in:-
7.User Settings
at Using Squawk option.
I don't install this panel every day not a big fan, but I support several on maintenance and do a full system check every service. I don't know all its features I wouldn't profess to know them all. From an engineering or user view point.
However all alarm systems work in similar ways, with there own slants on various functions and who can control them, the terminologies vary, but end of the day certain panels made for certain countries at certain points in time are designed to work in similar ways if they are to comply with any standards at the time whether they are used or not.
I find it hard to believe the panel wouldn't have shown a low battery in the log, I can believe it may not have been reported to the central station(should be though) or to the end user via voicemail or text as it can cost a lot of money having all notifications turned on.
If it wasn't enrolled / learned to the panel then it wouldn't do Jack but may well be illuminated. I know of many customers who try it themselves get in a mess and then call someone out to straighten it up.
The fact it was working and now isn't, would suggest that somethings changed (agreed), I would expect certain things to provide warnings (jamming, low battery, no signal for some time(supervision fault)), other things not to provide warnings (piezo fault, turning an option off, deleting a device) are just some examples of some these.