This is likely an issue in Chrome caused by an add-in or extension (or recent update). Please try the lectures in another browser. If the sound plays, you likely need to reset Chrome to default settings.
First, try restarting your computer. Weird problems with Chrome audio often disappear after a restart.
Second, please try the steps outlined on this page: https://www.makeuseof.com/windows-chrome-no-sound-fix/
Be sure that CoursePlus has not been blacklisted under 3. Check Chrome Sound Settings on that page.