Red Dead Redemption 2 introduced a bug where the exe would reduce the volumne on start to 19%.
Its now at least 4 month old.
So whats the problem? When you tab+alt out of fullscreen, to change your soundvolumen everytime you start the game, you have to rechange the graphics configuration as well.
I solved it by .net code i found on github which would run fur 5 minutes and would put the volumne up as soon as it ifinds the rdd2 process...