I've written a DOS batch script in my Windows 10 that can take multiple *mp4 files (and yes - with spaces in the name) and normalize the volume using mp3gain. (I tried MP4GAIN, but the trial period ended in like 1 hour). This one will process much quicker than MP4GAIN, I found out. I do this because I can specifically control ffmpeg and mp3gain every step of the way and do things that mp4gain or ffmpeg-normalize might not be able to.