This usually results because of a glitch on the Chromebooks. To resolve this issue please do the following:

1. Plug headphone into headphone jack
2. Click on the “settings” bar (bottom right – you should see clock, wifi, etc)
3. Click the “right arrow” besides volume
4. Click “Headphone”
5. Unplug the headphone and it should auto switch back to speaker (Internal)

Now when you plug the headphone back in, it should auto switch to headphone.