XMPP would be a better fit than activitypub.

Is this sufficiently exposed?

Yes, this is how I read it as well. The library would support to use a local model, but they decided to just send the audio data to Google.

There is nothing special going on. This whole project is just a bunch of python libraries coupled together to a cli tool. It uses the package SpeechRecognition to connect to the google speech recognition api: https://github.com/microsoft/markitdown/blob/main/src/markitdown/_markitdown.py#L691

Pretty uninteresting and a bit disappointing. Pandoc is a lot more interesting.

I like libreoffice, but converting audio files to markdown must be a pretty recent feature, for I never heard of it before being part of libreoffice.