Why does my sound change to boxes when I insert the microphone?

I have a headset over which I can hear wonderfully. But when I plug in the micro cable from the headset, the sound changes from headset to boxes.

(Laptop, Windows 8)

Why is that and how can I fix it?

Have you ever tried to switch the output back to headphones?

Yes. But that's usually the way he does it automatically, right? Have already tried all the entrances and exits.