Complex sentence: This explains why there is no sound when the flame is heating the gauze: all air flowing through the tube is heated by the flame, so when it reaches the gauze, it is already hot and no pressure increase takes place.
Split into simpler sentences: This explains why there is no sound when the flame is heating the gauze. All air flowing through the tube is heated by the flame, so when it reaches the gauze, it is already hot and no pressure increase takes place.