"How can Chell talk? Because in the games she's mute."

GLaDOS: That's basically the same question as how I can talk, because it's the same process. I just replicated the device that I use to talk, sized it down a little bit and made Chell go through a medical experiment to see whether it works for humans. Unfortunately it did.

Chell: Now I wouldn't call it unfortunate!

GLaDOS: It was for me...