I cannot believe this is a serious question. The plane does not need to make any adjustments to fly at constant altitute around a hypothetical round Earth. It has nothing to do with sound, it has to do with physics.
I can't believe you can't explain it!!!!!! The pilot would have to adjust the altitude periodically on a round Earth which by the way DOESN'T happen. And because you can't defend the round lie, the truth of the matter is that the Earth is flat.
[At point (A) the airplane reaches cruising altitude and pilot engages trim tabs, the ground below is a given 34,000 ft, upon reaching point (B) 500 miles later the ground below curved underneath around the so-called round Earth and gained altitude, now the airplane has to reach point (E), therefore altitude adjustment must be implemented or air routes are compromised.]
First off all trim tabs are electric and "engaged" even before the engines start up on the ground. Some flight control surfaces don't have trim tabs they can just be trimmed and those use a cable system. Either way you can't hear them. I will give you the flaps, but usually that is because there is a pressure restrictor somewhere in the system to prevent high hydraulic pressure from tearing up the gearbox that drives the flaps. Spoilers use the same system as the aileron, elevator, and rudder. Again, silent.
The reason an aircraft can follow the "curvature" of the earth is pressure. The autopilot maintains a constant MSL altitude by staying at a set pressure once it has reached up to its cruising level. Now while at this set MSL altitude, the aircraft may actually change it's AGL altitude as you fly along if the pressure changes. But the pilot only knows this if he has a radar altimeter that is on at altitude. Most don't. Otherwise as long as the main altimeter stays at the assigned MSL altitude it's good to go.
Whether FE or RE, this is related to density altitude. The plane will naturally settle where the lift and weight are at equilibrium.
This is partially incorrect. The first part is correct the second part is not. Lift and weight distribution have to be at equilibrium upon takeoff. Otherwise the aircraft will either not get off the ground or flip over once airborne and crash.