I am not sure I get it yet:
Do you want to make a chat between two ti-99 systems connected via serial/parallel interface? On such a system users could enter text on one system and it shows up (and the text is spoken out optionally) on the other system. Text entering and showing/spoken out vica versa between two systems.
Or do you want one system that simulates a chat between the user and the system? Where the computer part of the chat is speaken out. Like in Wargames.
The TEII speech options are really good for both situations. You don't need a disk system like.
Check the TEII manual, Point 14 Appendix C: Accessing Text-to-Speech in TI Basic and Point 15: List of allophones.
For a chat simulation with the computer you can optimize the speech quality of its answers:
The TEII manual explains how to set the pitch and slope, setting stress points, breaks, setting inflections.
In cases where this doesn't work you can setup Allophone speech and create your one words with its own pronouncation.
There was a TI Disk Software released for XB called Text to Speech which gives you about the same amount of possibilities like you would have using the TEII, but it needs a disk system and the routines take up a part of your avaiable ram:
Here is the manual of this Disk Software:
ftp://ftp.whtech.com/datasheets%20and%20manuals/Text-to-Speech.pdf
But I suggest using TEII