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 you don't want your phone to say ‘Hello Moto’ when it powers up:

You cannot adjust the volume level for this sound.
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: