unless you talk seriously quiet it should not be a problem.
is the RNS315 a factory installed radio in your car? or did you swap it in?
(reversing the mic wires will cause low audio levels and poor audio quality)
I messed with the mic's dba setting at one time, something in my failing memory suggests that the higher the value the lower my voice volume seemed.... but again my memory sucks, I could be thinking about something else, or even another car entirely. (getting old sucks)