This is a solved problem that has mostly to do with propeller shape. By creating 3D simulations of airflow you can come up with very quiet designs.
The reason it’s not done is because no one gives a damn. Planes and helicopters fly too high up and if you’re in one of these vehicles the sounds of the engine will be louder than the propellers anyway because they aren’t electric.
Relative quiet and actual quiet are different things - I don't believe the physics exists to displace the noise/vibrations generated; you can change the propeller shape and sizes, however that energy still needs to go somewhere, so you'll mostly just change where the energy's going and the frequencies and concentrations that are output. I'd love to be proven wrong.
The reason it’s not done is because no one gives a damn. Planes and helicopters fly too high up and if you’re in one of these vehicles the sounds of the engine will be louder than the propellers anyway because they aren’t electric.
The military however already has this tech.