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Sorry to butt in on the question, and for the long delay in getting to your question.
On most timers, you can adjust the volume.
If your timer volume is at maximum and is now very quiet, it's the speaker on the timer that has failed.
It could be replaced if you are good at soldering and electronics but in most cases, it would require a new timer.
However, it's worth trying to set the volume and see if you can increase it.
The following is the advise for 5 and 6 button timers fitted to a wide variety of makes of oven.
If your timer has 5 or 6 buttons press the timer button and the plus button together for 1 minute.
When it bleeps release the buttons and press the minus button once, if it is not loud enough, press it again while it is bleeping.
When it is loud enough press the manual or timer button to set it.
If this doesn't work, let me know the model number of your oven and I'll see if I have anything on your particular machine.
Hope this helps