Setting Your iPhone's Maximum Volume for Music and Videos
You can set a maximum volume on your iPhone for music and video playback. This prevents your volume level from accidentally being turned up beyond your comfort zone. (You only have one set of eardrums, after all.)
To instruct your iPhone to limit the loudest listening level for music or video:
Tap the Settings icon on the Home screen.
Tap iPod in the list of settings.
Tap Volume Limit in the list of iPod settings.
Drag the slider to adjust the maximum volume level to your liking.
(Optional) Tap Lock Volume Limit to assign a four-digit passcode to this setting so others can’t easily change it.