The Soundcheck WordPress plugin provides a new block type for the Gutenberg editor called Speakable Section for use in any post or page.
With the Gutenberg editor you can add a section of Speakable text the same way you would add a paragraph of ordinary text. The Speakable attribute is defined on schema.org as sections of a Web page that are particularly 'speakable' in the sense of being highlighted as being especially appropriate for text-to-speech conversion.
To add a Speakable Section to a page or post, first install and activate the Soundcheck plugin. Open the post or page using the Gutenberg editor and click the circled + icon to add a block. Select the Speakable Section block. You may need to scroll down to see the block for the first time.
Then simply type the text you want to mark as Speakable in the block. According to the recommendations by Google, you should strive for 2-3 sentences of content. Avoid punctuation and markup that would be difficult for a digital assistant to read out loud.