very little
Nicole,
While there are various ways of getting a computer to "read" text in a
simulated voice, they're all guaranteed to have your audience chuckling. If
that's the goal, then they're laughing WITH you, and that's cool. Just
remember that a little goes a long way. If the cute, geeky computer voices
aren't the point of the joke, they'll become a joke anyhow, only now the
audience is laughing AT you and we don't want that.
You can have PowerPoint record narration that you or somebody else delivers
into a microphone and play it back later. Maybe that's what you were after?
If so, check SlideShow, Record Narration on the main PPT menu.
That said, I wholly agree with what the others have said. An enthusiastic live
presenter will blow away a recording or cartoon or computer voice.