You might want to play around with this a little:
Luciano Notarfrancesco has ported rsynth to Squeak some time ago, and
now I've found some time to make a plugin primitive for the most
compute-intensive part.
Download http://www.httg.de/~hmm/squeak/SpeechSynthesis.zip, compile the
plugin (sorry it's not there yet, I will fix that soon) and use cmd-t to
let it speak selected text.

Have fun!
Hans-Martin