Volume normalization of audio track in MP4 video files

I am currently looking at how to "normalize" the volume of various video files without recompression, and documenting my findings here.

For MP4 video files, an easy solution appears to be AACGain, a command line utility that handles AAC audio. Simply run:

    aacgain /r *.mp4

and all MP4 video files will be normalized. The process is entirely reversible (using the /u command), and involves no recompression at all.