Formant based speech synthesis. There is also a program called "tex-edit" which apparently can pronounce English sentences reasonably using Macintalk.
MacinTalk doesn't run reliably on Macintosh's with new sound hardware under the lastest OS (System 7.1 w/HUD 2.0). More recent software is listed above.
By anonymous ftp from many archive sites (have a look on archie if you can). tex-edit is on many of the same sites. Try
Last Revision: 00:21 19-Mar-1996