Noise reduction benefit is marginal. Most of the noise comes from tires rather than the engine. At 30 km/h tires are louder than the engine, so an EV driving at 50 km/h, which is the typical city speed limit, isn't any quieter than an ICE car.
Noise benefit isn’t marginal. Central London average speed is 7 to 10 mph (11 to 16 km/h) so engine noise is significant. There is lots of idling traffic: traffic lights, taxis at cab ranks, buses at bus stops, everyone in a jam - removal of idle noise is particularly significant.