UniDescription is close to what you're talking about:
$1You might try reaching out to them to see if you can contribute. I have no idea how they go about doing everything and I haven't downloaded the app yet, but AD tours are a growing phenomenon (thank jeebus).
Here's a link to a Hadley Podcast episode about them: https://hadley.edu/podcasts/hadley-presents-conversation-experts/audio-described-national-parks