ToddAndMargo:
I gave up and bought a $15 wind up timer. At least I can hear it. Well hear it rattle for 4 seconds. It may have a bell sound, but I can't hear it.
Sometimes the old-fashioned ways are best. We got something like 25 years out of a wind-up timer before the bell fell off the back.
I just use the one in my phone, since it's nearly always nearby, I'll hear it go off (unlike the wind-up timer on the stove, which I probably won't hear at the other end of the house). I wish mine only did go off for a few seconds and quit.
Others have suggested things like gnome-clock (the desktop toolbar gadget, I believe), and the equivalent for other desktop systems.
I notice that Mate has a timer app that can be put on the toolbar, it pops up a dialogue when done, but is silent.