Hi agata101,
Thanks for using Apple Support Communities! I see you're having an issue with the sound when a set timer ends, and I'd like to help out.
You've done some great troubleshooting steps so far! Let's see if we can figure this out.
Are you having the same issue with an alarm? Or is it just the timer that will not sound?
If you hear no sound or distorted sound from your iPhone, iPad, or iPod touch speaker
If it's just the timer, verify the sound settings are set at Clock app > Timer tab > " When Timer Ends" tap and change the ring tone and set the volume.
If you have a custom ring tone, try using a standard ring tone, such as "Radar".
If you ask Siri to set the timer, do you get the timer sound?
Use the timer or stopwatch on iPhone
Verify if a bluetooth headset or other device shows connected at Settings > Bluetooth. If so, tap to disconnect that device.
If you're still having issues, verify that your iPhone is up to date with iOS13.5. Update your iPhone, iPad, or iPod touch
Thanks!