Unity's standard volume control uses a "linear taper", which does not correspond with human hearing. "Loudness", unlike the Audio Source's "Volume", will yield perceptually equivalent loudness changes for equivalent value changes. (Changing the Loudness from 0.2 to 0.3 will yield a similar difference in loudness between 0.7 to 0.8, etc.)
// I would not leave this in Update() for a release. // Put it wherever you need it, to change the volume in-game.
Why this works