Some voice commands (such as ‘What's up?’ and ‘What is the current weather?’) cause Google Assistant to speak in response.
To quickly silence a voice command response, tap 
To have your phone show responses to Voice Commands on screen and not speak them:

If your phone describes everything that you touch on screen, speaking your notifications and reading messages aloud, you might have turned on a screen reader. This accessibility feature helps people with visual impairments.
To turn it off: