Unless others are able to view or play that video on their browser to verify the problem this will be hard to know. How are you playing it in Edge and Chrome what are the steps your using? So others can see what is happening.

Thank you for your response.

I invoked it from Edge and Chrome (both under Windows) after putting it on a website and opening it by simple click. i did the same on Firefox and had my issue. I then tested it on Firefox under windows using the Firefox menu dropdown and "Open file" - the issue showed up doing that. I also tested it by directly opening it through Firefox under the Cpanel file manager on my hosted site and had the same issue.

The file was created by taking a simple powerpoint chart and adding a powerpoint recording over that slide, then exporting to 1080p video file (mp4).

Is there a way I can include a sample mp4 on this forum?

I will be releasing my work to the public very shortly, so I guess I'll have to tell them not to use Firefox on Windows or Android.

I'd still like to see if the issue can be fixed so I can remove that restriction.

It looks like no-one seems to know how to do anything with this issue. So, I've added a URL with a sample MP4 I created using PowerPoint. It shows the issue under Firefox, but is fine on Chrome and Edge.

http://thebagpipeacademy.com/Audio%20test/Firefox%20audio%20test.mp4

I'm still hoping this can be resolved. I have been using Firefox as my main browser for over 10 years, but I would have to change to Edge or Chrome because of this issue.

Any help greatly appreciated.

It looks like this issue is in the too hard basket. I'll wait another couple of days then close it off.