You will find the Loudness setting under preferences in the Sonos App

Loudness tries to compensate for the fact that the equal-loudness curve of human hearing is naturally more sensitive to mid range frequencies ( around 1000-4000 hz ) at low volume levels. The reason for this sensitivity in human ears is because this frequency range is where human speech normally lies. This allows us to better hear voices when the volume of sound or voices is low. It however also means that our ears/brain will naturally make an "EQ" setting for sound of low volume with mid range boosted. This will reduce the amount of bass and treble we register. Loudness compensates for this by boosting bass and treble at low volumes. It is a function of volume meaning that the boosting of bass and treble is reduced at higher volume.

In short this means that if you feel like your music is lacking bass and treble at low volume, then use loudness. Keep it mind that this will reduce the amount of mid-range in your music, which some would say reduces the fidelity of music. If you regularly listen to music for detail at high volume then we would suggest keeping loudness off. If you regularly use your speakers for background music at low volume then it may be worth trying this feature. For locations playing predominately instrumental music the noticeable difference in tonal quality will be nominal. However, at louder volumes some consumers report their music sounds more balanced with the loudness feature turned off.