Yes, only the sound samples of the allophones are needed for a close emulation of Speakeasy, as they are canned, not modifiable. You even doesn't need the Speakeasy device to obtain them, as they can be recorded simulating the chip on PC with the provided code.
I've been playing with Bliss, another Intellivision emulator written in Java, with the five vocal games that where released for the console. The sound is very robotic for actual standards, but on the other side I can understand clearly the words spoken. There isn't noise under the phonemes, and it aids to comprehend better the messages. Curiously there is some form of intonation on the games, aspect that I've not seen on the SP0256 documentation.
But.... if the complete chip can be emulated, then a new bigger Rom can be created with 256 allophones instead of only 64, with space to obtain "foreign" sounds for our country languages. I am only dreaming....