MP3Gain is a free utility that analyzes mp3 files and determines how they will sound to the human ear. It will then adjust the files so they have about the same loudness, without affecting the quality of the recording. You can use it in two ways, which are Track or Album Mode.
Track mode corrects the mix of unrelated songs to the level you select. While Album Mode corrects the entire album by relating them to the other songs on the album.