Mic input is only played in the left or right of headphones/speakers, not both sides!

Many users will think the problem is that their mic is "not in stereo" as they only hear it on one side, however, it is actually the opposite!  The problem is that you have chosen Stereo Input, when you want Mono, as you are only using one mic.

In your recording software, there will be an Input selection, to choose what is recorded, such as Input 1, or Inputs 1-2.

If your track is set to a Stereo Input, like Inputs 1-2, it is assumed you are doing a Stereo mic setup, using two microphones, placing Input 1 on the Left and Input 2 on the right.
Thus when you only have one input, recording in stereo, you get this issue, audio only on one side.

To correct this, be sure the Input to your track is set to Mono and your input number, such as Input 1, or whichever you are using.

This will correctly place the Input in the center of the stereo output, so it plays to both sides.

The example below is shown in Logic, but applies to other recording software too: