All iPhones and iPods these days have a feature called Sound Check. It scans all your music files and sets the playback volume so that they are all roughly the same level. iPhone/iPods also have an equaliser (EQ) function which lets you modify the sound of your music by boosting the bass, treble, and so forth.
Both these features by themselves work well, but when combined with today’s music it will cause issues with how your music sounds. Why?
A lot of music nowadays are mastered to be as loud as possible, thus reducing the dynamic range and possibly introducing some clipping in the process (the loudness wars, google it). Now while that itself is alright (well not really, but what can we do about it…), if you use the Sound Check feature and you have other quiet music in your collection, it will bump up the playback volume even more. Combine that with an EQ such as Bass Booster and your music will clip quite noticeably.
A good example of this is Metallica’s Death Magnetic. It was already mastered quite loudly and clipping is noticeably present. Turn on Sound Check and Bass Booster and it will clip badly.
So….. if you use Sound Check and notice distortion/clipping in your music, turn it off. It may not be your music.