This is an essential tool for being able to play all you mp3's without ever having to rush over and adjust the volume. It's amazing how much more enjoyable this makes the listening experience -- the songs somehow just sound "better" when their volumes are properly balanced with mp3gain. Forget alternatives like mp3 Rage, or iTunes "Sound Check" features, or others I can't remember the names of right now. Only mp3gain properly implements the ReplayGain model for adjusting relative volume of tracks to all have as closely as possible the same subjective perceived loudness. It applies psychoacoustic principles, chiefly a curve corresponding to varying human senstivitity to loudness at different frequencies, i.e. sounds roughly in the speech range of frequencies sound louder to us, that is we're more sensitive to them, than sounds further above or below this range. Anyway the long and short is that no other volume balancing solution on the Mac or Windows does as good a job (and I've tried lots). But note that you have to process each sound file before playing, which means setting mp3gain loose on your whole computer music library, letting it chug away for a few hours, and then every new track right away put it through mp3gain, or it will probably sound WAY TOO LOUD. The ReplayGaing model used by mp3gain will make most of your tracks quite a bit quieter, but that is to allow for tracks that actually do need the full dynamic range this leaves available. Trust me, it works extremely well on all types of music, allowing you to mix your punk and metal favourite tracks with acoustic folk, jazz, and ambient electronica and play the whole mix without having to touch the volume.

I only gave it 4 stars because (a) the Windows equivalent displays a scrollable display showing what adjustments were made to each track, which can be saved as a log, making it much easier to figure out if you have already processed a track; and (b) sometimes mp3gain won't process a track the first time through, for reasons I can't figure out, so if a track still sounds distinctly too loud, you may have to run it through mp3gain again ALTHOUGH this is based on my experience with version 1.8. I haven't tried 1.9 yet (am just about to install it).

Mega=thanks to the developer for updating the program. Now if only someone would produce a Mac version of WaveGain, which lets you apply the same adjustments to WAV files on a PC, essential for creating mix CD's -- also much finer grained adjustments can be done with WAV (or AIFF) files, since mp3's are limited by their nature to increments of 1.5 dB. Which is good enough for general listening, but sometimes not quite fine enough. Meantime I use Virtual PC to apply WaveGain to WAV tracks before I burn them to a mix CD -- a hassle but worth the trouble.