I didn't have to do anything to add text to narration. I had read many medium articles about how to do it with third party services and downloading to sound cloud etc, but realised the article already had a button to play audio. After drafting and before publishing, there is a box to tick/untick in settings if you don't want text to audio narration. So there isn't anything else to do. It enables people who don't have time to read or maybe vision problems to listen instead. Surprising audio isn't counted for income, like reading is.