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.