I think audio is an excellent feature to add. As a librarian I can tell you many people prefer listening to written material rather than reading it.
I love to read aloud so I would truly love it if I could narrate my Medium writing using your audio feature.
I write mostly poetry on Medium. The auto narration feature is not as effective at reading poetry aloud as it is reading prose. I think this is because the AI voice is looking for “clues” in the text such as commas or periods that tell the voice to pause slightly or stop. But in poetry there often are no such uses of punctuation. Thus the computer generated voice runs on without any natural language pausing. It sounds a bit awkward to the listener; but even with that, I do think that adding the audio feature to Medium is a good thing and I hope it will continue for both poetry and prose.