Unity's standard volume control uses a "linear taper", and humans hear on a roughly exponential scale. "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.