A common issue with Chromebooks is that the default audio setting get changed to speakers instead of headphones.
This is usually caused by the default audio out being accidentally set to speakers when headphones are plugged in.
Below are instructions on how to solve this issues.
1.) With headphones plugged in, using the trackpad or a mouse, left click down on the bottom right of the screen where the time, wifi, and battery icons are located.
2.) Next to the speaker volume slider click on the > arrow (See Image 1.1 below).
3.) Under Output change the Click Headphones this will change the setting back to headphones.