Audio normalization is the application of a constant amount of gain to an audio recording to bring the average or peak amplitude to a target level (the norm).
Then, the question is, how can we unify the size of the sound? Please refer to this tutorial: http://www.videoconverterfactory.com/tips/normalize-audio.html
This tutorial shows you how to use a freeware program called MP3Gain to normalize MP3 files without losing audio quality. Instead of resampling every file made by some programs, this lossless technique, called Replay Gain, uses the ID3 metadata tag to adjust the "loudness" of the track during playback. Resampling usually reduces the sound quality.