Is there currently (2019/10, or in the HTML "living standard") a "standard" way to alert assistive tech users that audio content is available for a given article/page of text?
E.g., the opposite of a podcast transcript. Instead of providing text for audio, I am looking for a way to alert assistive tech that audio exists for a given text.
Is this a thing?
Note: I see that the WCAG Checklist states:
If the audio or video is designated as an alternative to web content (e.g., an audio or sign language version of a web page, for example), then the web content itself serves as the alternative.
And perhaps that is my answer? That providing an "audio" version of something implicitly links it to the text provided? Am I understanding that correctly?
If you are looking for a particular markup to signal alternate content, as far as I know there isn't something that specific. So the passage of the spec that you have found says it all: the best and the simplest is to put a link in a sensible place.
Now, more deeply into the fundamental question, is an audio alternative for a text useful?, I would like to answer no in general.
As a blind or visually impaired, we all have assistive technologies that read the text for us, this is called screen reader. We usually take time to configure a voice with its settings (speed, pitch, etc.) to our liking. People having other handicaps, like dyslexia, also have assistive technologies of that sort.
There's also apps to read text on mobile outside of the browser, for example VoiceDream Reader on iOS, with the same possibilities to configure the voices to use. Apps like VDR aren't exclusive to visually impaired, people who read books while driving can also use them.
Finally, it's quite improbable that your audio be used instead of our own ways to read text. It's becoming really useful only if there's added content compared to the raw text: For example accompanying music, sound effects, and/or text played by an actor or at least read by an human...
If there isn't that added content compared to the raw text, then providing an audio just for the text is totally useless. If you are providing an audio version because the original text isn't accessible, then you'd better make the original text accessible first.