I assume you are certain it isn't your hearing going bad - sometimes the high-pitched tones of beepers become inaudible without properly functioning hearing aids, or the ability to hear certain tones disappears altogether. This is easily checked by asking someone with known good hearing if the beeper is working. Hearing problems can occur at any age, so don't dismiss this too quickly.
If everything else works normally, it is either the beeper (usually a piezoelectric disc on the controller board or attached to the board with wires, but sometimes a small speaker) or a fault in the controller board's beeper circuit. The first thing I'd do is check the disk and any wires for cracked or loose connections. (CAUTION: unplug the oven before removing the cover.) It's usually located on or near the controller board behind the key panel. Generally, piezoelectric discs are about 1/2" to 2" in diameter, have a brass rim with a silver plated ceramic center, and may be enclosed in a drum-shaped black plastic housing with a hole in the center to let the sound out.
If the disk is connected to the controller with wires and a removable connector, unplug the connector, clean the pins and test it. Piezoelectric audio devices use very little current, but a dirty or oxidized connection will still keep them from working. It is possible that the disk itself has mechanical (spring) contacts, and the silver electrode plating at the contact is tarnished. In that case, gently clean the silver with a soft white pencil eraser. Do not over-polish; the silver film is very thin.
Is is possible, but difficult, to repair a cracked solder connection on the disk. You must use an acid-free solder flux and silver-bearing solder (3% silver recommended), and you must work quickly using an iron at the correct temperature for the solder or you will completely detach the silver film from the disk.
If necessary, you can find replacement disks at electronic component vendors such as Mouser Electronics. The primary specification is the frequency (musical pitch), given in Hz or kHz (Hertz or kiloHertz). If you don't have that number, getting a disk of the same diameter will probably be close enough. You'll want one with wires or PC leads already attached. piezoelectric Audio Devices Mouser