Do the videos have audio? I have tried Safari and Chrome but no luck. I see there is captions so wondering if they just don't have audio.
All videos in the course have audio, backed up by closed captions. I have checked again, and they work fine on Chrome, Safari, and Firefox.
I hope you have resolved the problem already. Just be sure your browser can play sound with other websites (the same one you are using with the course, in the same OS). Sometimes it's disabled or not working at all.
I had this same issue on my 2011 iMac with latest updates and no browser extensions. The audio worked for many chapters and then all the sudden it stopped working. I could play youtube in that same browser in a different tab. I tried restarting the computer, but no luck. It still does not work in Safari. When I switched to Chrome on that same computer it started working again.
When I'm at work I use either Chrome or Opera and it works in both. My work computer is Windows 7.
There is also a browser called Brave. You might give that a try if Opera, Firefox, etc. don't work.