The built in voices can be a bit boring, and no fun at all.. However, if you click on the System Voice drop down, then choose customise you will find a list full of all the available voices. Now, scroll down all the way to the US voices, and the past the normal ones. You should reach a section which looks like the one to the right.
Selecting these voice files add’s a little bit more novelty and variation to the Speech engine.
In this list you can also download some more voice files for most languages which don’t ship by default with OS X.