Navigate to the MP3 player's "Settings" menu and select the audio equalizer, often referred to as the "EQ." If you don't find this option in the root of the "Settings" menu, look for a subsection entitled "audio" or "music." Adjusting the equalizer will allow you to customize how your MP3 player outputs different frequencies of sound.
Ensure that the volume on your MP3 player is set to max if you are outputting the music through powered speakers; this will help to reduce distortion. Control the level of the volume with the speakers from that point forward.
Use your headphones on another MP3 player. The issue may not be with your MP3 player at all, but rather with your headphones. Alternatively, you can use another set of headphones with your MP3 player. Replace the headphones if they are defective.
Transfer an MP3 with poor sound quality to your computer and play it with Windows Media Player. You may find that your MP3 files themselves are the issue. If so, try deleting your music from your MP3 player, re-ripping your MP3 library, and transferring the new MP3s to your player.