When confronted with a live microphone, people tend to alter their voices to sound more "radio-like." If you're one of those people, stop it. Trust yourself to be yourself. "Radio voices" are cornball and inauthentic. You'll have a better shot of making a lasting impression by simply being you.
Radio was a big deal when I was growing up in Chicago in the 1980s. What struck me back then was how not-radio-sounding some of the biggest stars of the day were. Steve Dahl, Eddie Schwartz, Terri Hemmert: All were polished, but none had that fake, jivey delivery that has become the cliché, "cheesy radio announcer" voice.
The only difference between talking on-mic and off-mic is the presence of a mic. That one variable shouldn't push you to change you who are and how you communicate.