Audible MR low speed warning
OOW,
In my humble opinion your passenger was totally over reacting.
I would be worried about his reliance on a warning which, lets face it, could go wrong.
I have my own audible warning devices, I call them ears. My back ups are my eyes when checking the rotor speed dial.
Tune yourself into your aircraft and you will be able to tell if things like RRPM are not where they should be.
Hmmmm......Zen and the art of helicopter flying.
It could be a winner.