How to Solve Volume Problems on an iPhone
Volume issues that occur when listening to music or watching a video on your iPhone are often caused by the sound check feature being turned on, or due to the iPhone's volume limit being set to a low level. Both volume limiting and sound check can be found in the iPhone's iPod settings, which includes EQ configuration to change the equalizer settings for songs and videos to maximize sound quality. According to Apple, a passcode may be required to disable the volume limit if one was originally created when volume limit was enabled.
Touch "Settings" on the iPhone's home screen and select "iPod."
Touch the "Sound Check" tab to disable the tool. The switch will be gray and display "OFF."
Click the "Volume Limit" tab, and click "Unlock Volume Limit."
Enter your volume limit passcode using the on-screen keypad, and slide the button on the volume limit bar all the way to the right.
Tap the "iPod" tab, and tap the "Settings" tab. Touch the "Sounds" tab, and slide your finger against the button on the volume bar to turn your ringer volume all the way up.