One of the most popular features of the iPhone is that it functions both as a phone and an iPod that plays MP3 audio files. MP3 files can come from many places, such as iTunes, artists' own websites and other sites designed to allow a person to purchase and download music.
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Sound Check is a utility that allows you to set all of your iPhone audio tracks to a uniform volume. Each iPhone comes equipped with this utility within the phone's iPod settings.
Varying Track Volume
Each track in your audio library may have been recorded using different methods and may have different volume settings. Using Sound Check can allow you to listen to all of your tracks without the need to adjust the volume with each new track.
Enabling Sound Check
Turn on your iPhone, then tap the "Settings" icon. Scroll down until you see the iPod icon and tap it. In the iPod menu, Sound Check is the second setting from the top and you can simply move the slider to the right to turn it on.