Volume normalization

different volumes, it may be useful to store volume information about a file in the tag so that at playback time, the volume can be dynamically adjusted.
A few standards for encoding the gain of an MP3 file have been proposed. The idea is to normalize the average volume (not the volume peaks) of audio files, so that the volume does not change between consecutive tracks. This should not be confused with dynamic range compression (DRC), which is a form of normalization used in audio mastering.
Listeners who prefer to experience music as it was intended to be heard on the original compact disc may prefer to not use volume normalization, because the average volume of each track was set intentionally by a professional mastering engineer.
The most popular and widely used solution for storing replay gain is known simply as "Replay Gain". Typically, the average volume and clipping information about the audio track is stored in its metadata tag.
Reference : http://en.wikipedia.org/wiki/Mp3