In 1978, Texas Instruments introduced the first single-chip linear predictive coding speech synthesizer, the TMS5100 (also known as the TMC0280). The Solid State Speech Synthesizer for the TI-99/4A used the TMS5200 (TMC0285).
I’ve scanned the manual for the Speech Synthesizer. (You can click on the picture on the left to download it.) On page 4 it states:
Plug-In Speech Modules
For topics that require a specialized vocabulary, Plug-in Speech Modules can supplement the Speech Synthesizer’s resident vocabulary. These small modules plug directly into the Speech Synthesizer itself. To access these additional vocabulary words and phrases, the Speech Editor Command Module must be inserted into the computer console. Your vocabulary list then includes the Speech Synthesizer’s resident vocabulary and the vocabulary of the Plug-in Speech Module.
Each of the Plug-in Speech Modules is sold separately, and several of these modules are planned. Ask your retailer for the latest list.
According to Wikipedia,
The original intent was to release small cartridges that plugged directly into the synthesizer unit, which would increase the device’s built in vocabulary. However, the success of software text-to-speech in the Terminal Emulator II cartridge cancelled that plan. Most speech synthesizers were still shipped with the door that opened on the top, although very few had the connector inside. There are no known speech modules in existence for those few units with the connector.