Does anybody know a solution?
I think you are might be if not thinking about then needing something like Normalization option from Media Player Classic applied to your web browser
My answer to this is: apply normalization to What-You-Hear on the level of the operating system
I do it with my X-Fi Surround 5.1 Pro by the means of option
Audio Control Panel > THX TruStudio Pro > Enable Smart Volume
But there are supposedly purely software solutions of that kind, but I have not researched that topic yet
Or do you want to replaygain video files dowloaded from YT?
Zerow, thank you for your helpful reply. Sorry, i really was not exact enough. I was thinking of an add-on for Chrome or other browser; but the MPC idea seems interesting, i shall take a look.