Go to the Control Panel then click on Sound. You should see the Realtek HD Audio Manager icon there. Click on that. When it loads up, double click on the two jack images below "Analogue" and you'll get a choice of headphone or line out. Choose headphone.
I had mine set to line out so I could use some external speakers and had totally forgotten about it. It might also be a default or something that just happens when you use certain devices? I'm not a techy, I wouldn't know. But I definitely fixed mine