Answer
On updated web, phone browsers, iPhone and Android, newly generated Gemini TTS narration includes Read Along automatically. It covers Narrate on human or AI messages and explicit spoken answers requested in Text or Audio. You can use it in chat, Media & Files and AI Media & Files.
CC is outlined when off and filled when on. It remembers your choice on this device. Turning it on while audio is playing opens the fullscreen Narration reader at the current position. Autoplay alone does not open the reader; CC off keeps ordinary playback.
The reader highlights whole paragraphs, headings, quotes, table blocks and list items as they are spoken. It is block timing rather than word-by-word highlighting. New supported narration times top-level headings separately. Human recordings and music keep ordinary playback without CC. Older app builds still receive readable text and a playable MP3.