Caution — after watching Kartik's video, turn down your volume to a normal level before watching mine. (I wish there was some sort of automatic tool I could share so that all our videos would have normalized audio. If anyone knows of something...)

I ended up using something called ffmpeg-normalize with a command line of ffmpeg-normalize --sample-rate 44100 -t -14 [filename] -o [out_filename] See https://superuser.com/a/323127/91637