A lot of these frontends have an option for using OpenAI's TTS API, and some of them allow you to specify the URL for that endpoint, allowing for "drop-in replacements" like this project.
So the speech generation endpoint in the API is designed to fill that niche. However, its usage is pretty basic and there are curl statements in the README for testing your setup.
Anyway, to get to your actual question, let me see if I can whip something up. I'll edit this comment with the command if I can swing it.
In the meantime, can I assume your local text files are actual `.txt` files?