If a video will play but there is no sound, it's likely the issue there is something going on with your device.
To check if that's the case, please try watching a YouTube video in the same web browser you're using to view the course. If the sound is not working on a YouTube video, the issue is probably a setting on your device.
The first thing to check is that you haven't hit the mute button on the video. The mute button and volume control is circled in the image:
If you use Bluetooth devices, it's also worth checking if headphones or a speaker might be receiving the audio. A fast way to test this would be to turn off the Bluetooth on your computer to disconnect any such device.