It seems to me that such a thick, thick fog provides a blanket or wall of silence against which sound echoes.
Here in Melbourne, we can experience thick fogs - especially in or close to the mountains. Upper Gully is in the foothills of The Dandenongs which are thick with temperate rainforest. The further up the hills you go - on some days - the thicker the fog. My picture was taken early one July morning at the Upper Gully railway station: an early morning blanket of silence which could echo with the footsteps of each new arrival to the platform to wait for the train.