Programs developed by Pro-Software.
Analyze, improve, or standardize your MP3 files different in quality, volume, bitrate, and so on.
It takes a bit of practice to tweak each track properly. But have the added benefit that the voices of the singers will have a better tone and the audience will notice it. By that way, if you change the BPM (beats per minute) of a track and also tweak the pitch in order that the harmonies of two songs match, you get a perfect music mix.