I'm not interested in spending 15 bucks for this, but I want to enlighten potential users of the possible interest of such an app: this one deals with the loudness level of files according to current standards (i.e. LUFS which is the loudness unit used in recording studios and broadcast) — which the free mp3gain app mentioned by wickedsp1d3r doesn't.Nevertheless, none of them are displaying a sensible value! They use a maximum target value of 100dB which absolutely makes no sense, as the maximum level of a digital audio file is 0dB... While the free one actually CONVERTS the file although it pretends not to (check with anything like Invisor Lite and you'll see that the 'Writing library' changes), this one doesn't seems to, but it looks like it rewrites the actual audio content anyway (hence the very long time it takes...). In both cases, if you choose a target too high, you'll get clipping distortion. In conclusion, it's not just a ReplayGain that is applied, so I personally would stay away from any app of this kind except in very particular cases...