As a cyclist I am always being told I should carry a bell (twee) or a klaxon. Why? "Get out of the way you dozy ****wit" does marvels for my stress reduction programme and has been known to have a spectacular effect on the hard of thinking... One guy lobbed a styrofoam cup of Starbucks Ripoff Coffee clear over his shoulder (and the lees over his Times) after wandering out into the road in front of me...
Back to aeroplanes - marvellous idea; Shorts exhibited a Skyvan at Farnborough a couple of decades back with the "Skyshout" system on board... Can't recall what was said but you could hear it clearly over the whine of the Garretts and the wonderous words of the Master Commentator, John Blake.
Just hope none of us ever need to use it in earnest...