Attaching the sound to RPM is not the right way to do this. While Prop RPM is a component of how much sound is generated by the prop, just as much of the sound is determined by the prop pitch. A perfect example of are turboprop aircraft powered by the Allison 501D / T-56 and its later RR AE2100 derivative. These engines and props are direct-drive turboprops, thus they operate a lot like a piston engine. The only thing that changes with throttle movement is the prop pitch to take the additional torque and thus provide more power. The difference in sound is massive as the big Aeroproducts Constant Chord (CV-580, C-130A-E & L.188), Hamilton Standard Hydromatic (C-130H, P-3, and CV-5800), or Dowty-Rotol (C-130J) props move to accept the power of the 4000+ HP engine behind it.
Additionally, the ENGINE tone on a piston or jet aircraft will change independent of prop RPM as additional power is added. It tends to get a deeper tone even with the same RPM being provided by the prop. Consequently, when reduced towards (and to) idle, the engine tone tends to "lighten" as the load is removed from the crankshaft.
BTW, comparing cars to airplanes never works. Two totally different theories behind operation, two totally different methods of propulsion being dealt with, so just because something goes one way in a car small-bore, low compression gas engine, don't expect it to translate to a large-bore, high compression aircraft piston engine in any way.
you are right regarding the different sounds of Prop and Engine (Combustion, various mechanical and Gas charge and discharge noises). But in terms of compression, we should not mix up the compression ratios and the reasons because of choosing different ones for car and airplane engines. The compression ratios of airplane engines are often much lower than the ones of car engines. There are different reasons for this. One reason is that smooth running is very desirable for aircraft engines to relieve the bearings and gears. Another reason (applies for the early GA engines) is because they need to be able to be started by hand, and by windmilling in flight.
If you compare the compression ratio of an average natural aspired aircraft engine (O-360 5,9L: 180HP: 8,5) and a typical natural aspired car engine (2,8L VR6: 174HP: 10,0) the car has higher compression ratio. This is just an example, of course you need more parameters to get the higher specific power output of the car engine, and of course I know that car engines see a MUCH lighter load spectrum over their lifetime. If you use a car engine with exactly the same load spectrum of an aero engine, it would come apart pretty soon
Charged engines, no matter if cars or airplanes, need the lower compression ratio because of the "knocking" (detonation) limit. The RR Merlin V12 for example was 6,0 compressed. What stresses an engine is the BMEP, not the compression ratio. The lower the BMEP, the longer the engine will last.
I hope, this was not too off-topic. The Info above is partly taken from my own experience and daily work during turbocharger development, and maybe someone knows more about aircraft engines and can add something.