It's been a long time since I worked with speech in AppleScript (or, indeed, worked with AppleScript at all). But if I remember right, the basic command is just:
say string
where string is what you want the machine to say.
Typical AppleScript genius. I assume there are probably more arguments and such that you can give the function, but that's the most basic way of doing it.
Or it was, when I did anything with it. I think.