Here's the guide I found!

https://writingcooperative.com/how-to-add-free-audio-narrations-to-your-medium-stories-a509e2a9cad#:~:text=Step%203%3A%20Add%20Audio%20Narrations%20to%20Your%20Medium%20Stories&text=If%20you%20want%20to%20add,'Add%20Audio%20without%20editing'.

It's a Chrome extension that pairs with Medium. I record on a cheap microphone I got at the local music shop!