Captured the behaviour in my stream test here: https://www.youtube.com/watch?v=hu35slPXrHY&html5=1. (You don't have to watch the whole thing ;-)
I Alt+Tab out at ~0:30 in and Alt+Tab back ~0:40. Sound mutes as ArmA goes into the background, and when I Alt+Tab back you can hear the wind moise and ambient structural noises restart, but the main engine does not. Have only tested as pilot but can test in other slots if it is necessary.
Initial testing was done in MELB MH-6A, but have tested with vanilla AH-9 and RHS CH-47 with same results.