fwiw I've never had the slightest trouble hearing the low RPM horn - and a good job too. I've flown with Bose A20 and Lightspeed Zulu and PFX, and the same applies to all of them. It is LOUD, as indeed it should be.
I was wondering about the remark concerning the way to hold the collective/throttle too. I've always been taught to hold it just behind the throttle, with just thumb and forefinger on the throttle.
The place where I did my heli primary lost an R22 to a soloing student who heard the low RPM and instantly went into autorotation - good. But she rolled the landing, end of R22, though luckily she was unhurt. My instructor (and hers) said that he'd noticed she had a tendency to over-control the throttle...