Is the reason they only respond in text and not audio (TTS) the same why 15.ai has been temporarily down for an update since over a year?[0]
Given the silly cartoony nature of the game, the current implementation is working fine for now. We are of course always looking at options when they present themselves, and as costs go down with the advancement of tech, it's more possible we'll see our characters speaking as well someday in the near future.
That's our answer from a design and product standpoint at least. Hope it's helpful!