Why do some aircraft sound different than others?
There are many reasons why aircraft sound different from one another. The noise characteristics of an aircraft depend on aircraft type, weight, thrust, speed and airframe configuration, among other contributing factors such as ambient conditions like weather, proximity of structures/objects in the area, environment, and the aircraft angle relative to the person hearing it.
For example, different types of aircraft have variations in performance features, engine design, and aerodynamics. Some aircraft make more noise than others because of the type of engines they use. Newer aircraft and aircraft engines typically are quieter as a result of improved designs and technology. Many times, an aircraft will sound "deeper" or "throatier" (lower frequencies are more prevalent) when it is loaded heavily compared when it has a lighter load.
Additionally, where you are standing in reference to the aircraft and the aircraft's direction of travel matters. Many factors including reflecting surfaces, distance, aircraft type, aircraft engines, throttle position, aircraft speed, the aircraft's angle and position in relation to you and your own hearing can affect the sounds and frequencies you hear. Generally speaking aircraft sound levels are loudest the closer you are to the aircraft and when you are positioned behind the aircraft.
Proximity to an aircraft and the environment where you are standing affects the volume and frequency of aircraft sounds because the sound you hear could be reflecting from surfaces around you. Sounds from aircraft are greatest when you are behind the aircraft and off to the side slightly.
For more information about the science of sounds, check out this video: Sound Measurement 101