VolumeDroid auto adjusts volume levels when you plugin and unplug headphones.
VolumeDroid will automatically adjust your phone's music, ringer, notification, alarm and system volumes to your desired level, as you plugin and unplug your headphones. NB if you are in silent mode then the ringer and notification and alarm volumes won't apply.
After installing the app you will need to start the app for the detection to be started and to set your volume preferences. The app can also be temporarily disabled in the settings. There is also an option to run a regular check of the volumes to ensure that the volumes are reset to the desired volumes if changed temporarily.
Due to difficulties in Android for users to get the volume setting bar all the way across, both the full bar and the position before will give you full volume.