The Screen Reader can work in 4 different ways:

• Reading the specific part of the text selected by the user
• Using a 'Play' buttons embedded directly inside contents
• Reading the text of selected elements when hovering them
• Reading a default text in the page as soon as the 'Play' button is clicked with no text selection, based on CSS selectors

In the latter case it works using a parameter based on CSS selectors to match the text to read in the current page.

The Screen Reader ships with a preconfigured set of CSS selectors to match the most common Joomla templates, but if this doesn't suit with your template you can add or replace specific CSS selector to match the text part of your template that you want to be read by default.
A precious tool to identify the HTML structure and the CSS selectors to use in this process, is the Chrome developers tool or Firebug for Firefox to inspect easily how your template is formatting elements.