Adjust the volume balance and mono audio on iPhone
You can adjust the audio volume balance between the left and right channels. You can also use Mono Audio to combine the left and right channels into a mono signal played on both channels. This way you can hear everything with either ear, or through both ears with one channel set louder.
Go to Settings > General > Accessibility.
To adjust the audio volume balance, drag the Left Right Stereo Balance slider.
To combine the left and right channels, turn on Mono Audio.