I had this problem and fixed it the following way. Through trial and error.
1 - Set my microphone to listen & Was testing audio on discord (One of them in plenty).
2 - Disconnected mic from headset and plugged it directly to the pc, the mic was working.
3 - Then re-plugged it to headset and wasn't working.
4 - Disconnected the headset single 6mm jack from the splitter and re-attached it twice and now it works.