Some questions are graded as correct by default and will need manual review. This is the case with open-ended questions and audio questions. Below is how to perform a manual review:

First, you will need to navigate to the game results screen, which can be accessed through the control panel at the right side of an active game, or from your reports screen.

Answers which can be manually re-scored, will have an icon at the top right of their answer cell. If you have not reviewed this answer, it will show a pencil icon:

Answer cell for an audio question type

If you click on this icon, it will show a modal which you can just the scoring for their answer:

The icon will change to a check after you have reviewed it by pressing the save button on the modal.