so, if i understood well :
TI99 don't have TMS5220 but the previous version, TMS5200, labelled CD2501E-NL
and it's a huge difference in sound quality !
Unfortunally, i didn't found the CD2501e-NL datasheet to see if it's possible to remplace the original CD2501E-NL by the latest TMS5220 & roms, but i found a partial answer on WIKI :
TMS5220 (AKA CD2805E?): Improved version of the TMS5200, pin but not function compatible (has new LPC tables and a new chirp table); used on (later) Apple II Echo 2 cards, (rumor) on the very last run of TI 99/4A speech modules.