Do nacelles around aircraft engines help in reducing the engine sound?
Take a modern turbofan: The tips of the fan run at approximately Mach 1.5. We know from unducted props that supersonic tips make them very noisy. The same turbofan without the shroud would be extremely noisy, too. Acoustic liners on the inside of the fan shroud have major contribution in suppressing this noise source.

But there is more: The nacelle also makes it possible to put an intake in front of the fan which helps to equalize flow conditions over the cross section of the intake and to accelerate or slow the flow of air to just the right amount for the engine. Without the intake, the engine would suffer from frequent compressor stalls and surges, so the nacelle in effect widens the operating conditions and avoids those noisy incidents, too.

Another important source of noise is the nozzle: Here the hot, fast gas from the engine is ejected into the surrounding flow, and the mixing of both creates most of the engine noise. Note that noise increases with the sixth to eighth power of jet velocity, so the most dramatic noise reduction is possible with higher bypass ratio engines.


Diagram from the Stanford web page linked above. The jet velocity is in ft/s.

By serrating the trailing edge of the nozzle, this mixing process can be slowed down somewhat and the noise level can be reduced.

Without the nacelle the engine would create much less thrust and more drag, and bigger, more powerful engines would be needed. Bigger engines would create more noise, too, so this is also an indirect way how nacelles help to keep noise down.

A last noise reduction contribution to mention here is the capacity of the nacelle to contain the shrapnel from a failed fan blade. Fan blade failure is all but silent, but if the blade fragments would escape at high speed, the noise of what follows next would also be considerable.