Normal breath sounds originate from the larynx .When the sound leaves the larynx it travels down the trachea and then divides when the airway divides. Some sound must be transmitted through the lung parenchyma but most travels down the airway. Eventually the sound travels along airways of different lengths and therefore becomes out of phase. Next it arrives in the respiratory bronchioles and alveoli and then gets transmitted through the chest wall to your stethoscope. The fat layer filters out much of the high frequency sound (above 4 kHz). The resulting sounds are much softer (because the sound has effectively been diluted throughout the whole of the lungs). There is no gap between inspiration and expiration (because all of the sound has become out of phase and therefore 'filled in' the gap. Finally, the first third of expiration is now the only part that is audible because the latter two-thirds are much quieter.
Reference: Chamberlain
