Read the iTunes FAQ...

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When you move music from your PC into the iTunes Library, the quality of the sound can be affected. To maximize sound quality, change the Importing Setting to match the Bit Rate of the file you are importing. For better audio quality, the higher the Mono or Stereo kilobits per second (kbps) and the larger the file size. To change the audio Importing Setting in iTunes:

1. From iTunes, click Edit, then select Preferences.
2. Click the Importing tab.
3. Select an encoder name from the Import Using drop-down menu that matches the file type that you are importing.
4. Select the quality setting from the Setting drop-down menu that most closely matches the Bit Rate of the file that you are importing.
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