I really can't say that I've had experience doing voice-overs professionally unless you count my ability to read aloud in classes using expression and a variety of tones. Usually, when it comes to someone to read a story, they would always choose me as their reader because, I guess, I read quite well. Considering that I am still in school, I really don't have time to audition for voice-overs though I would really LOVE to if the opportunity comes.
I really didn't "train" myself to do voice-overs professionally yet, but I do get some training at school when I read and try to portray the different characters in that story.
My additional skills include acting which is also a dream of mine, if doing voice-overs don't work out.