If you have a hearing impairment, the iPhone and iPad feature accessibility options that make certain features more usable. One of them is a mono audio option. Since stereo sound offers distinct left and right channels, someone with a hearing impairment in one ear may miss something. Enabling mono audio will ensure that all sounds go through both ears at all times. You've just got to enable mono audio in Settings to start using it.
How to enable mono audio on iPhone and iPad for those with hearing impairments
Launch the Settings app on your iPhone or iPad.
Tap on General.
Tap on Accessibility.
Under the Hearing section, turn On the option for Mono Audio.
That's it! Now whenever you use headphones with your iPhone or iPad, you should notice that all audio comes through both channels. If you favor one ear over the other, you can also adjust the slider underneath the Mono Audio option in order to increase volume in a particular ear.