MP3Gain is a wonderful freeware that makes normalizing sound of audio files a piece of cake! It is a free sound amplifier with which you can amplify any MP3 files with just a single click!
MP3Gain does not just do peak normalization, as many normalizers do. Instead, it does some statistical analysis to determine how loud the file actually sounds to the human ear. Also, the changes MP3Gain makes are completely lossless. There is no quality lost in the change because the program adjusts the mp3 file directly, without decoding and re-encoding.
The text quoted above from the official site says it all.
Just drag/drop or add files/folders in the MP3Gain window, set the target volume (dB) and press the “Track Gain” button, that’s all! You’ll find uncompromised quality of the MP3 file for your targeted volume!
This freeware works on Windows XP, Vista, as well as Windows 7.