What does MP3Gain do?
MP3Gain automatically adjusts mp3 audio files so they all have the same volume
Do you mean that MP3Gain normalizes mp3 files?
Yes, but MP3Gain doesn’t use “peak amplitude” normalization like other “normalizers” do. Instead, MP3Gain uses David Robinson’s Replay Gain algorithm to calculate how loud the file actually sounds to a human’s ears.
This is one of the most used free programs to adjust the volume of MP3 files.