MONITORING
Under some headphone or earbud monitoring situations, you might want to adjust the AUDIO BALANCE (Left & Right)
In iOS 9.3, head to Settings > General > Accessibility. Scroll down to the Hearing section and you'll see a slider for left/right balance. Move the slider to the side where you would like to increase the volume level.

For many audio sources, stereo sound features different sounds and levels in the left and right channels. With mono sound, you'll hear the same sounds in both channels. Above the volume balance slider on the Accessibility page in Settings is a toggle switch to enable Mono Audio.