After the update All update cars have driving in the Cockpit ,a very soft engine sound, almost inaudible , Please fix Reiza. Other cars are good in sound
Please check, if your windows audio settings are set to the correct amount of speakers that you're using. Otherwise surround sound could cut-off your engine sound that's coming from behind for these cars.
The Gr.C are sounding normal here and i saw a stream yesterday after the update, also with perfectly normal sounds aswell. Could you look around in your car with left&right look button, does the engine sound appear?