I only hear quzz when making a phone call using SIM800L module
The microphone input on the SIM800 is ment to interface an electret microphone. The input sees a difference between the voltages on MICP and MICN as its signal.

But the way it is connected now (R17 between MICN and C6) there will never be a voltage difference between MICN and MICP (other than the BIAS voltage ment to feed the microphone).

I guess the quzzz sound you hear are minute fluctuations in the current that the BIAS generator drives through R17. As there is no other input signal the module turns up the audio amplification and the quzzz gets audible.

To solve it: Disconnect R17 from C6, and connect it to ground. 
Now MICN will be static, MICP will 'move' with respect to it and the module will see input.

This modification creates a DC connection from MICN to ground. The datasheet is blank on what could happen if you do that. I guess with R17 it's ok, but it might make the BIAS generator and/or input stages unhappy. If the sound is not perfect yet, try adding a capacitor (say 100n) in series with R17.

It could still be that the PWM output is too loud for the mic input, and the sounds get distorted that way. In that case increase R5 to attenuate the PWM output further.