Using a digital pin out for a buzzer
All PWM outputs of microcontrollers that I have seen have been digital.  What would a analog PWM output even mean?  Maybe allow you to set the high and low voltage levels?  I have not heard of such a thing being integrated into a microcontroller PWM output, probably because there wouldn't be much use for it.

Your question is therefore based on a false premise, and can't be answered directly.  To drive a speaker to make a fixed tone, the PWM output of a micro can be appropriate.  These are always digital.  The circuit then takes this digital signal and uses it to apply and not apply voltage to the speaker, or in some cases flip between two voltages.