Hi Jens,
Thank you for letting us know about the issue.
For your information, most browsers don’t allow playing unmuted audio/video without user interaction first. So, it may cause your issue.
Kindly see the Autoplay policy of browsers here: Autoplay policy in Chrome - Chrome Developers
The best practice to overcome this issue is inserting a button on the first slide. The user must click on that button to start the course.
You can clear Disable [Click or Tap to Unmute] Notification check box on the Export to HTML5 dialog to remove the message. Then, you can insert a shape (along with the button I mentioned above) for your own message if it’s necessary.
Hope that it clarifies.
Thuy