I've heard this one a lot because of the unnatural computerized voices that comes with the phone. I would just be recreating the "memos" app. Also, you wouldn't be able to see the highlighted word as it's being spoken since I can't parse the text with the custom audio you input.

What I think will solve these problems is AI voices. I've heard some very natural AI voices. Once they are more accessible by the general public, it would be a no brainer to add it the app in place of the built in voices. I think this will be sooner than people realize