Julian, in an effort to help Jasper learn to read, created for him a wonderful alphabet book, which can actually talk to him! Touching the letters, the letters will name themselves. Running a finger across the word makes it say the word. Touching the accompanying picture has it make a sound.
For the letter "J", Julian put in a picture of Jasper, and captured him saying his own name.
Jasper, on hearing it, claims that it isn't him saying it.
He's fooled by the fact that when you hear a recording of your voice for the first time, it doesn't sound like you, because you are used to hearing your own voice through your bones as well as your ears.