Highlight portions of text as it is read

I have encountered a problem and I hope someone can offer a solution.  I have a slide with tabs. When the user selects a tab, it takes them to a portion of text and it is read aloud using text-to-speech. The customer would like a smaller portion of the same text to be highlighted as it is read. Has anyone found a solution? I can add a transparent yellow shape to highlight the area and align it to the voiceover, but I can't figure out a way to get it to move along with the voiceover. Does anyone have a suggestion?