SDL_Mixer master volume

I need to add a gradation of sound in my application. But I can not figure out how to do this with the out-of-the-box functionality.
Maybe someone knows how to do this? Or should I add the API for the master volume in SDL_mixer?

So, I made a patch for SDL_Mixer 2.0.4 to add a master sound level.
master_volume.patch (3.7 KB)