iTunes 11 for Windows: Adjust the volume
Adjust the volume of music and video
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Change the volume for all songs (like turning the volume knob on a stereo): Use the volume slider at the top of the iTunes window.

The maximum volume is limited by your computer’s volume setting.
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Adjust the volume of a particular song or video: Select it, choose File > Get Info, click Options, and drag the volume slider. iTunes uses this setting each time the song or video plays.
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Have all songs and videos always play at the same volume level: Choose Edit > Preferences, click Playback, and select Sound Check.
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If you’re using remote speakers connected to an AirPort Express: You can use the volume controls on the speakers as well as the iTunes volume controls. To use only the speakers’ controls, choose Edit > Preferences, click Devices, and select “Allow iTunes audio control from remote speakers.”