Pwm for 2 head sets and a pc(tower) using arm7 m 3 micro controller

I have a project and I got really stuck!!
The project is about air plane and headsets were the pilot and the co-pilot has
a button each
If the pilot presses the button he can speak through the microphone found in the headsets here the co-pilot and the tower are able to listen but none of them can talk when the button is pressed. Therefore when any of the three is talking the rest can not talk even if the button is pressed. when programming this project I’m required to use PWM pulse width modulation because we don’t have enough DAC digital to analog convertor.
How shall this be programmed?
Which duty cycle or period?
Any tip or advice or even a code that can help !!
I really appriciate your help.