QMP3Gain is a simple application that allows you to fix all the MP3 files that have issues with their volume.
The interface of the program is pretty simple: you add tracks by clicking on the 'Add files' button, or you can add an entire folder with the 'Add folder' button. When the main window shows that they have loaded, click 'Track analysis.'
When the analysis is finished, the application will show information about what needs to be done. What needs to be modified shows up in red, while the tracks that have already been analyzed and approved show up in green. The tracks that show up in blue are the ones that have been modified by the program.
Once you know which tracks you need to fix, you can normalize the volume by using the 'Track gain' option. Or you can customize it manually by listening to it and modifying the 'Constat gain' option. Then you're set to go, with QMP3Gain having normalized the volume of all your MP3 files.