The reason I had to take this route is that Adobe Audition’s Match Loudness batch process does not export in m4a format. I routinely use Match Loudness to level the volume of the assorted audio files I’ve collected into a musical compilation. Audition’s Match Loudness function is the best tool I’m aware of for accomplishing this essential task.
There are several options -
There is an unofficial [u]ReplayGain plug-in[/u] for Audacity. This isn’t the “real” ReplayGain as it “permanently” changes the volume instead of writing the ReplayGain tags.
There is a free 3rd-party loudness analyzer called [u]dpMeter 4[/u] that works in Audacity. (It just measures loudness so you have to make the changes manually.)
[u]MP3Gain[/u] and [u]AACGAIN[/u] also make “permanent” changes to the actual volume and they work losslessly without decoding-recording the compressed audio.
Apple’s version of ReplayGain is called [u]SoundCheck[/u] and it’s built into iTunes.