Its projection and diction.
You need to be able to project your voice...most guys do that by going baritone...buts its not always necessary.
The key, though, is diction. Being precise with pronunciation when saying things like the letter w. Most pronounce that dublyou (for lack of a better way to make the point) but a radio announcer will spend time (I know I did) learning to distinctly say double---you)...its important for call signs on the east coast...and while training myself to say it wished I was out west where they start with K)
As far as accent, the training was to "speak like you are from nowhere". So, for those with accents, which is most of the country, that equates to speaking as a mideasterner (ohio, illinios, indiana) where, in general, the accent is fairly neutral. Even there, though, there are tendencies to partially drop final vowels (Ohigh as opposed to O hi O) and so saying the radio voice is a mideastern accent is also a bit problematic.
All I know is what little hick accent I had left after moving back from europe was completely gone after my first 6 months in radio. And that made me happy.
