Adding inflection (Voiceover)
If you don't want to sound as if you're reading copy (if you want to instead sound as if you're speaking conversationally to the user), consider using more inflection. Inflection is a change in the pitch in your voice, moving up or down the scale as you talk.
Lack of inflection pretty much defines the reading voice. If you read a paragraph of text in a normal reading voice, you won't hear much inflection. But if you listen to a real conversation, or especially if you listen to actors on TV, their voices move up and down the scale with a lot more inflection. It seems the more emotion you add to what you're saying, the more inflection you end up including.
According to Old School, inflection is key to keeping the attention of your audience:
If you know anyone whose voice never rises or falls, who drones on and on in the same monotone, you know how soon you lose interest in what he is saying. The voice that holds your attention conveys emotion and interest by change of pitch or inflection. Old and Sold
One reason inflection keeps the audience's attention is because inflection communicates emotional investment in what you're saying. When you believe in the content and it matters to you, your words become emotionally invested and inflection naturally follows.
When I listen to Robert Segul on NPR, I can hear the inflection in his voice. Part of the problem with inflection, though, is that you can't just start inflecting and assume it will make your voice more believable. If you inflect in an unnatural way, the result is corniness. It's easy to fall into an annoying emphatic rhythm, or to overdo inflection to the point that it draws attention to itself.
An unnatural inflection is almost worse than lack of inflection. You've seen the equivalent in text when someone can't stop writing with ALL KINDS of emphasis that, well, you just find .... S-U-P-E-R ANNOYING. You have to inflect in a believable way.
Exactly how you inflect in a believable way is an art. When you're closing a thought, your inflection goes down. When you're raising a question, your inflection goes up. Dan Levine recommends that you start reading about in the middle of your pitch, so that you have room to inflect. If you start too low, you can't go lower on the scale, so you're stuck.
You can also twist a word here and there to break out of a reading rhythm. You'd be amazed at how you can pick -- almost at random -- any word in a sentence and twist it to free yourself from a monotone reading rhythm.
As you focus on inflection, listen as you talk. Make note of how you change your voice, how you pause and move up and down the scale. Listen to the inflection in other people's voices as they talk. As you start to focus on inflection, you'll begin to hear what is believable. It's something we unconsciously ignore.
One note. I know plenty of people who speak in a flat, monotone voice as their normal pitch and inflection. If that's the case (and it's partly the case with me), consider opening your mouth and getting into what you're saying a little bit more. You might have to get creative and imagine yourself in a situation. For example, rather than imagining yourself talking to a user about the calendar feature of your app, which might bore you and leave your delivery flat, imagine yourself in a courtroom before a jury, or something similar.