The programmer who was doing the DECTalk work talked about this some:
> I ask Schnee if Buchler’s source code could be reconstructed from a copy of the software.
> Not exactly, she says. It might be possible to induce the machine code to dump out a database of every phoneme it had ever uttered — all the tiny pieces it had used to sing Armenian and Lithuanian and Egyptian trope — and then, if you were very patient and very skilled, you might be able to write code to piece them back together.
> It could be done, she says. But it wouldn’t be quick or easy. And it wouldn’t be the same program Buchler wrote.
There's also a sound file of their implementation at the bottom of the article.