Closed Captioning displays text of the audio content of a video on the screen. You can customize how the text appears on Prime Video and other supported video content.
Just say "Alexa, turn on all captions" or follow the steps below on your Alexa App or Echo Show:
Select Settings.
Scroll down to Accessibility.
Select Captioning to enable and select preferences.
You can also tap on the Audio and Captions button in the top right corner of the screen when a Prime Video is playing.
Closed Captioning is available on all Echo devices with a screen