The manual doesn't doc which voices are available. But I believe the syntax is just using the name like so:
> say -v Karen Hello
I don't have access to my Mac right now but I found this list from here:
- American English: Jill, Samantha and Tom
- Australian English: Karen and Lee
- British English: Daniel, Emily and Serena
- South African English: Tessa
There is also other languages as well.
UPDATE:
say -v ? spits out:
MacBook-Austin:~ Austin$ say -v ?
Agnes en_US # Isn't it nice to have a computer that will talk to you?
Albert en_US # I have a frog in my throat. No, I mean a real frog!
Alex en_US # Most people recognize me by my voice.
Bad News en_US # The light you see at the end of the tunnel is the headlamp of a fast approaching train.
Bahh en_US # Do not pull the wool over my eyes.
Bells en_US # Time flies when you are having fun.
Boing en_US # Spring has sprung, fall has fell, winter's here and it's colder than usual.
Bruce en_US # I sure like being inside this fancy computer
Bubbles en_US # Pull the plug! I'm drowning!
...
If you do not see the voice you are looking for when you do a say -v ? you can install more.