nostr:npub1knzsux7p6lzwzdedp3m8c3c92z0swzc0xyy5glvse58txj5e9ztqpac73p

ffmpeg has a speech normalization filter:

https://ffmpeg.org/ffmpeg-filters.html#speechnorm

You can tell ffmpeg to normalize the audio stream using "very strong and fast amplification" and pass the video stream through unchanged, with the command below. It's quite fast, doesn't affect the file size much, and the audio quality seems fine.

ffmpeg -i input.mp4 -af speechnorm=e=25:r=0.0001:l=1 -c:a aac -c:v copy -y normalized.mp4