I haven't tried MacMP3Gain yet, as I just upgraded from an OS 9.2.2 machine to a newer OS X 10.1.5 iMac. Yeah, I'm living in the past, but at least I've finally made the leap to OS X. Bit MacMP3Gain requires OS X 10.2, and won't run on this OS X 10.1.5 machine.

I have, however, used the Windows version of MP3Gain extensively for the last few years -- running it in Virtual PC on my old OS 9.2.2 iMac, since there is no version for OS 9 or earlier. It is an essential tool! It brings all your mp3's to a similar subjective loudness (using some psychoacoustic modeling). What makes it better than iTunes' "Sound Check" feature (which also works quite well I think) is that the mp3's retain their altered loudness no matter what they're played in -- even burned to a CD as mp3's and played on my DVD player (which supports mp3's). I wouldn't even bother making a compilation CD without a tool like this -- it makes a huge difference to one's listening pleasure to have the songs play with relatively matched volume.

Note that it doesn't change the dynamics within the song, only between songs. Sounds great!

Another goody on the Windows side is WavGain, which preceded mp3Gain and operates on a similar principle, but with considerably finer gradations of loudness than the 3 dB jumps used by MacMP3. It's what I have been using for making compilation CD's from original (non-mp3) sources, again running in Virtual PC. I wonder if an equivalent is available for Mac OS X....