I feel you're overcomplicating things and/or are describing some different phenomenon. The point isn't the question whether it's the sound, the light or the air

I feel you're overcomplicating things and/or are describing some different phenomenon. The point isn't the question whether it's the sound, the light or the airplane itself that reaches you first. The question is which direction is the sound coming from. Imagine a plane flying 3 km high, circling around your location in a circle with radius 4 km. In other words, the plane is consistently sqrt(3² + 4²) = 5 km away from you. Let's assume the speed of sound is at a constant 343 m/s in this scenario, so sound takes 5e3 m / (343 m/s) = 14.58 seconds to travel from the plane to you. The direction of the incoming sound that you detect will change all the time, at the same speed as the direction of the plane itself, but it will lag behind. The sound that you hear at each moment is the sound that the plane generated 14.58 seconds before, and you detect it as coming from the location the plane was in at that moment, and not the current time. Your eyes (or a camera, or a radar) detect the plane from one position, your ears (or a directional microphone) detect it from another, older, position. All that is true independently from the speed of the plane, be it subsonic or supersonic (except when the plane is flying very slowly, or if it's a hovering helicopter: in those cases the sound is still delayed, but the location it's from is hardly changed or not at all). Strictly speaking the same happens with the visual image, but since light is so much faster we can neglect the delay it causes in every-day situations like this.