[u]ReplayGain[/u] works by adjusting the volume at playback time so it has to be supported by your audio player. The files are pre-scanned and a ReplayGain “tag” is written into the file telling the player how much adjustment to make (up or down). I use Winamp (with ReplayGain) as my audio player, but winamp is “outdated” no longer being developed.
If you have an Apple device, [u]Sound Check[/u] works similarly so if you have an iPhone or iPod (and iTunes) you can simply switch-on Sound Check.
WaveGain and MP3Gain “permanently” change the loudness of the file so they work with any player.
There is a [u]ReplayGain plug-in[/u] and it does (optionally) alter the loudness of the file, so of course that will also work with any player.
MP3Gain will not download on my computer for some reason.
Hmmm… Works for me… I just downloaded mp3gain-win-1_2_5.exe from MP3Gain Downloads
If you have MP3s, MP3Gain can adjust the volume without decompressing/re-compressing the audio. If you load an MP3 into Audacity (or any “regular” audio editor) it gets decompressed. If you re-export as MP3 you are going through another generation of lossy compression. You don’t always hear quality loss but you should avoid doing that if possible.
If you are ripping CDs you can rip to WAV, then run WaveGain or use the Audacity ReplayGain plug-in and then convert/export to MP3 once, as usual.
The limitation of MP3Gain is that without decompressing you have to change the volume in 1.5dB steps. But, that’s still pretty-good and it usually gets you within 0.75dB of the target.