Voice too "authoritative?"
The solution will depend on what the survey respondents meant by "too authoritative". For example, it might mean any of the following:


your talk is pitched at too high a level, so people feel dumb when
they don't understand, and are too intimidated to ask questions 
your style is argumentative 
one or two people did ask questions, which you didn't answer clearly 
your style is pompous 
you sound overconfident
you state your opinions as fact
...and so on


I assume the survey was confidential, so you can't go back to people and ask them what they meant. So I suggest you ask a few colleagues who are good speakers what they think you need to improve on.


  My voice is very deep and, in order to project well, I tend to speak
  loudly (no microphones).


A deep (low-pitched, bass-y) voice projects farther than a high voice. You may simply be speaking too loudly for the room.