OS X Command Line Utilities
This article was nice, but it indicated that you need to change the system preferences in order to change the voice.  It turns out that you can pass the -v flag to use a different voice, or "-v ?" to get a list of names.So you can do    say -v Samantha Help, I'm trapped in this castle!

or    say -v Princess Help, I'm trapped in this castle!

But wait, it gets even better: The Mac comes with a number of voices for other languages, such as French, Hebrew, Chinese, and Arabic.  And it turns out that the voices will work in English as well, using that country's accent.  I just tried it with a few different languages, and was amazed by how well this worked, within reason:    say -v Sin-ji  "Hello, I am Chinese.  What do you want to eat?"
    say -v Thomas  "Hello, I am French.  What do you want to eat?"
    say -v Carmit  "Hello, I am  Israeli.  What do you want to eat?"

And yes, I should get back to work...