I often watch multiple videos simultaneously (don't ask why). Today I was trying to watch video1 and video2. The problem is not the overall volume, but rather the volume difference. If I raise the master volume then the second video is too loud, despite being on the lowest setting.
The question 'How to increase the volume on certain YouTube videos' is not relevant because the answers refer to increasing the overall volume of the computer rather than changing the volume of the video itself.