You don't need to impersonate your settings. You narrate them.
A roommate of mine constantly is hearing narrated Agatha Christie novels, and each of them is invariably told by a single person. The speakers (either male or female) tend to adjust their voice for narrating individual parts, but a female voice does not become staggeringly male or vice versa: there is just some consistency and a recognizable intent and character. And inflection and loudness does not sway wildly either. It's an auditory sketch, that's all.
If you feel unsure about, get any old audio book and listen to it. Speakers stay within their comfort zone while indicating their intent. That's all.
It's important that you as a DM stay within your comfort zone.
You are the one holding the threads of the game in your hand and that's what you need to maintain. The terror of the final encounter with the foe is not in your voice, but in your narration and his stats and his effects. Refrain from anything that might make you lose your cool or feel weak: you are the universe.
