Oh wow. Wowowow. This actually worked:

Open the developer console and paste this in:

  var context = new AudioContext();
  var videos = document.getElementsByTagName("video");
  var audioElement = context.createMediaElementSource(videos[0]);
  context.destination.channelCount = 1;
  audioElement.connect(context.destination);

The audio will now be in both channels.

From: https://mikedombrowski.com/2019/03/fix-youtube-mono-one-ear-...