When the headphone is used, some users experience extremely low volume even you maximize the volume by using the volume icon on the menu bar.
Here how to fix the problem.
1. Under the Menu, search for the application called "Open PulseAudio Volume Control"
2. Select "Headphones" on the port:
3. Move the cursor to the right to increase the volume
4. You also need to re-adjust the volume on the volume icon at the menu bar if the sound becomes staticky.
Hope the tip helps.