I personally have been very frustrated that my Ring cameras only, some times, fail to make an alert sound on my Apple cell phone.
Possible Solution that I discovered: TURN OFF BLUETOOTH.
This sort of makes sense because with bluetooth on, the cell phone is set to send audio to your bluetooth headsets and not your cell phone speaker. Obviously, most of the time you are not wearing a bluetooth earphone so you should remember to go to cell phone Settings and look down to find Bluetooth and view the toggle switch to run Bluetooth on or off:
On if wearing bluetooth headsets. Note that if you are wearing headsets you should hear Ring alerts.
OFF if you want to hear Ring devices from your speaker.
NOTE: The Apple left side slide switch must be ON to allow the Apple cell phone to provide you the Ring audible alerts.