From Text-to-Speech to mp3 on a Mac

Try to execute the following in a terminal:

say -o ~/Desktop/say.aiff  "Hello. I'm a Mac"

That will save the spoken text "Hello. I'm a Mac" to ~/Desktop/say.aiff. See the manpage of say (execute man say in a terminal) for more information and other file formats. You can also easily convert the .aiff-File to an mp3 with iTunes or a console based encoder like lame.