Jur, Normally when one rides to above 27 km/h (17 mph), the wind noise buzzes one's ears. If you have a quiet chain or a singlespeed or fixed gear, the road noise from the tires is louder than the sound of the chain. In such instance, the rider feels a steady progression along the road with mainly road noise from the tires.

Using a fairing in the pictures seen in this thread, the chain noise is collected by the fairing instead of being dispersed and re-directed back to the rider (the concave shape of the fairing the rider faces causes this). So the rider hears a mechanical sound of the bike's chain instead of just the rushing of air in their ears. In normal riding, the level of my ears is above the fairing. In this position, I hear both the sound of the chain being reflected by the fairing, road noise and wind. When I duck below the fairing, I only hear chain and road noise with zero wind sound.

There is some truth to the fairing providing some degree of protection from small stones and road debris kicked up by other motorists. However, the fairing would be impractical for daily use I think because it is easily scratched (polycarbonate made by 3M). If the edge of the fairing touches the fork of the bike, any ruts or rough patches of road will cause the fairing make a noise caused by the contact of the fairing against the fork. This can also be an irritant.