Works on M4As, unlike the open source. Not so much on MP3s.

I’ve used the old Windows open source version of MP3Gain for years. There’s a freeware Mac port, MP3Gain Express, that works well for MP3s, but doesn’t do M4A files. I’ve only run a few M4As through this so far, but they came out fine. MP3s with large gain reductions, however, didn’t fare so well. There are burst of garbled audio in the music.

This program appears to be unaffiliated with the open source programs of the same name. I assume it’s a port without credit, which of course would violate the license terms. The support site is unmaintained. I did not try to contact the author.

tl/dr, use it on M4As, not MP3s. That’s worth $3 to me.

nottooloud about MP3Gain - Audio Normalization