The iPhone's music player application has a function that limits the volume of music and videos to either a manually set maximum or to the level recommended by the European Union. This limit is enforced regardless of the volume of the file you are playing or how much you press the volume up control; while helpful at times, it can prove annoying if you are using a particularly quiet set of headphones or are in a very noisy environment. To disable the volume limit, you need to modify your iPhone's settings.
Open the "Settings" app on your iPhone and tap "Music" to open the configuration for the Music app. On iOS versions older than 5, both the app and the setting are called "iPod" instead.
Tap "Volume Limit" to open the configuration for the volume limit function. If the switch labeled "EU Volume Limit" is set to "On," tap it to switch it off.
Drag the slider labelled "Max Volume" all the way to the right. You can only interact with this slider if the "EU Volume Limit" switch is off.
If both the switch and the slider in the volume limit settings are greyed out and you can't interact with either of them, changes to this setting have been restricted either by you or by somebody else. In the Settings app, tap "General," tap "Restrictions" and enter your pass code when prompted. From here, either tap "Disable Restrictions" to remove all restrictions from your iPhone or tap "Volume Limit" and then select "Allow Changes."