There are a few settings to check if you are unable to hear the videos. First, check that your computer is not muted, you can often check this through your settings or by selecting the speaker icon at the bottom of your screen. If your computer isn't on mute, and you still don't hear sound, check the video sound. This looks like a set of increasing height bars at the bottom of your video viewing area. You should see blue on those lines - if they are all grey, your video is muted. You can click on those lines to unmute.