Quadravox has a phonemes based syn system. It uses recorded phonemes and controlling the replay produces the speech, just as the SPO256.
Along with the chip, they has a windows IDE to help in compiling the words. I think they have teamed up with Parallax, I saw a ad for a BSII voice board.
You should look at the Quadravov site, http://www.quadravox.com/. They have alot of info along this line that you may find usful.
Good luck,
Bill