If you use MPV, you can pass it ffmpeg filters, and ffmpeg has many ways of compressing the dynamic range. See [1] for the set of audio filters. Here are the settings I use [2]. If you can't run your video through ffmpeg, I guess you could run your audio signal through a physical compressor if you use external speakers, but that can get quite involved.

[1]: https://ffmpeg.org/ffmpeg-filters.html#acompressor [2]: https://github.com/ruuda/dotfiles/blob/fff8d1d905767e85b57d7...