If you can, get copies of the stacks "Talk To Me!" and "HyperMacinTalk." They include XCMDs that convert words and phonemes to MacinTalk speech. Talk To Me! is a ShareWare stack, and HyperMacinTalk is freeware, so they are both usable only for non-commercial purposes.
As for the latest version, MacinTalk 3 version 1.4 is the most recent as of July 1998. For more information on MacinTalk, read Apple Technical Note 268, titled "MacinTalk: The Final Chapter". Also check Technical Note 19, titled "How to Produce Continuous Sound Without Clicking".
The following Tech Info Library article can help you find the Tech Notes mentioned here:
Article 24493: "
Apple Tech Notes: What They Are, Where To Find Them"