More from the article Sveta linked to:
"The voice you’ll be hearing is the one you set in System Preferences > Dictation & Speech > Text to Speech. On this screen, you’ll find a number of different male and female voices. You can add more by selecting Customize. You’re also able to change the speaking rate, but you’ll need an app like Dictater if you want better control over the feature."
Macworld has an article on changing the voice to the 6 preloaded voices, as well as adding additonal ones "...in the Text to Speech tab of the Speech system preference, click on the System Voice pop-up menu. You’ll see the voices, divided by gender. (In Lion, the female voices are listed first. In Snow Leopard, the male voices are first.) To see what each one sounds like, select it, and then click the Play button. You can use the slider bar to adjust the Speaking Rate.