From Windows Media Player 11:
If your songs play at different volume levels, you can have the Player level (also called normalize) the volume for you so that you don't have to manually adjust the volume when a new song starts. The Player does so by reading a volume-leveling value in a file, and then adjusting the volume accordingly during playback. Volume leveling is only available with files that are in Windows Media or MP3 format and contain a volume-leveling value. This value is automatically added to the files that are created during ripping. In addition, you can add this value to files on your computer that you have added to your Player library. Adding the value does not change either the quality of a file or how the file plays in other players or portable media devices. To turn on volume-leveling, do the following:
Click the arrow below the Now Playing tab, point to Enhancements, and then click Crossfading and Auto Volume Leveling.
Click the Turn on Auto Volume Leveling link.
To add the volume-leveling value to a file, play the entire file. The Player will then level the volume for that file whenever you play it in the future.
To hide the settings, click the Close button in the Enhancements pane.
(Optional) To add a volume-leveling value to files, do the following:
Click the arrow below the Library tab, and then click Add to Library.
Click Advanced Options, and then select the Add volume-leveling values for all files (slow) check box.