Still not buying the idea that the low RPM warning is not loud enough to hear even with radio traffic. Radio volume levels, if set to a proper conversational level, do not in any way mask the warning horn, regardless of the type of headset you are wearing.
However, it is possible to have a weak or malfunctioning warning horn, which is why testing it is a before takeoff checklist item. I've personally experienced a weak/failing horn and it's definitely not a smart idea to fly like that.
P.S. you know you are a well trained Robinson pilot when your car's low fuel warning buzzer causes your left arm to snap down
