Pan the Mic Input to Center, then!
Select the Phones Output on the third row(click on it so it becomes highlighted), then check the first row whether your inputs are in Mono or Stereo.
If your Mic is connected to ,let's say AN1 and you see AN1/2 in the first row, that means that they're in Stereo.
Click on the "Tool" symbol on the right side of that Input and the "Settings" Tab will appear. There, you will see a lit "Stereo" Button.
Click on it to deactivate it. Now, you will have 2 Mono Inputs. Pan the one you have your Mic connected to, to Center!
Hope, this helps!