There are two ways to turn the sound on/off.
1. Simply click on the audio icon, at the bottom of any game's instructions screen:
2. Or simply click on the "Menu" button during a game and then click on the sound option:
Please note that the "Menu" button is not available on purpose during an assessment task.
If you still cannot hear sound, make sure that your audio system is turned on or enabled, and finally check that the mute option is not enabled.