Hello!
In your application drawer you will find the "Google App" click to open. At the top left you will see three horizontal bars, click to open the side panel. Next click on the "Settings" button. Here you will find "Voice" option, click it. Here you will find "Speech Output", click and make sure it is selected to "ON". Next go back to the previous "Settings" page and you will see at the top "Google Assistant", right below it select "Settings". Here scroll down to section Voice and Speech and select "Speech Output" to ON.
This should fix that. As a note, by default if your sound is turned OFF you will not hear Google Assistant Voice Output.