for the interface, i think it is this website. :
http://sites.google.com/site/analogvocoderinfo/speak-spell
I've taken a lot of info from there.
That is an interesting article - though it doesn't give what the chip ID/number is...
as i know in the S&S, it is a TMS5100, which is quite different.
TI made several different voice chips - and those PDF files are all I have; and one of those chips was referenced by a collectors site. Maybe the best thing to do would be to open your S&S up, and see what chip you have? Maybe TI used different chips on different revs...?
I know that it is now a collector, but sometime you can find cheap ones on ebay.
Well - sorta-kinda now - but in probably a decade or two, once everyone else has kitbashed and circuit-bended (or just plain ripped the guts out for a robot!) all the old Speak-n-Spells, those that are still in "pristine" condition will be worth something. Probably not enough to retire on, but maybe enough to make you wish you hadn't modded yours...
Your choice though - I could be completely wrong about the future, too!
;D