In the latest alpha version (0.58.105348) the engine sounds seems quieter for nearly about 50 percent. If you listen clearly to your environment, you can still hear the popping in/out sounds of incoming/outgoing vehicles.
Dear Devs, please fix it and bring back the immersion, that ARMA III deserves.
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In the latest beta version (0.70.106872) the engine sounds are still too quiet.