In what way(s) will making an airplane electric help to make it "quiet"?
You are right, propeller noise is dominant in case of GA aircraft, but not all propellers make the same noise. Much depends on blade loading and tip speed - making the propeller larger and letting it turn more slowly will improve efficiency and reduce noise. This has worked very convincingly for model aircraft over the last 30 years. GA engines are normally ungeared, so their propeller speed and diameter are the same. Switching to a geared IC engine like a Rotax 914 not only makes the engine lighter but also the propeller less noisy. Switching to an electric motor, which can produce high torque at low speed, will allow to exploit this effect to the fullest.

Other sources of noise will profit from electrification, too. The exhaust noise will be completely avoided, and airframe vibrations will be greatly reduced. Radiator noise will fall away, too. It is a small source of noise, but every bit counts.

What will be left is the noise of the slower propeller(s) and aerodynamic noise. Now landing gear retraction would be the next big step in noise reduction.