Iphone audible warnings missing

HI. Did you check if you have do not disturb mode enabled?

Use Do Not Disturb on your iPhone, iPad, or iPod touch - Apple Support

If this option is disabled try this:

Try to reset your device. This won't delete your data stored on device.

• Press and hold the Sleep/Wake button
• Press and hold on the Home button
• Keep holding both buttons until the display turns off and back on with Apple logo on it.

Alternatively, you may go to Settings - General - Reset - Reset All Settings

Do you have the mute switch enaged? It's the slider above the volume buttons. If you can see orange, it's turned on. Slide it toward the front of the phone to turn it off.

Glad it helped.