Why do the aircraft's engine noises change during flight?

Aircraft need more power to climb than to descend, in the same way that a car needs more power to go up a hill than down one. The aircraft has more than enough power to climb, so shortly after take-off you will notice a change in noises as the power is reduced. The pilots also control the pitch angle of the propellers for various stages of the flight and as they 'change gears', this can also be heard in the cabin.