How to Adjust the Equalizer on Your iPhone
You can adjust the equalizer settings on your iPhone. Some equalizer settings emphasize the bass notes (low end) in a song; other equalizer settings make the higher frequencies more apparent. Consider changing your equalizer settings depending on the type of music you're listening to. Rock and roll and dance music tends to sound best with heavy bass, while classical and jazz might want a bit more treble. You don't have to guess; your iPhone also offers preset equalizer selections for different kinds of music.
Tap the Home button on the front of your iPhone.
Tap the Settings icon on the Home screen.
Tap iPod in the list of settings.
Tap EQ in the list of iPod settings.
Tap different EQ presets (Pop, Rock, R&B, Dance, and so on) and listen carefully to the way they change how the song sounds.
When you find an equalizer preset you think sounds good (or just matches the kind of music you're currently listening to), tap the Home button and you’re finished.
Note that you don't have to follow all the rules with an equalizer. If you don't enjoy your music with lots of bass, for example, you can select a setting that de-emphasizes it and just keep it there, no matter what kind of songs you play. Conversely, if you like to feel your music in your chest, you can choose one of the bass-heavy settings.