I’m not on a Mac right now, so I can’t test, but this page suggests you can do
say -f script.txt -o greetings.aiff
to load what should be said from script.txt and save the audio output as greetings.aiff. You can then convert it to mp3 using lame with
lame -m m greetings.aiff greetings.mp3
Definitely try the different voices. 😀