I have seen the same thing, except that if I stay on the browser page receiving the webcast, the problem is not apparent unless the meeting lasts longer than an hour, and usually not even then.
If I switch from the receiving browser page to do something else on my computer, the problem noticeably appears. The longer I am away from the streaming tab of the browser, the worse it gets, and it becomes obnoxious quickly. This happens whether I am viewing a different browser tab, a browser tab in another instance of the browser, or if I am doing something else on my computer while the webcast is running in the background (like checking on other concurrent webcasts, monitoring the servers, working on a LibreOffice word processing or spreadsheet file, taking meeting notes in a text file, etc.).
The audio is always delayed after the video, never the other way around.
The same solution works for me. Reloading the page syncs the video and audio again.
I suspect that it has something to do with the way my browser and my computer process the video and audio signals. I suspect that part of the problem is that the servers provide a video signal separate from the audio signal that starts in sync, but that there is nothing further from the server to keep them in sync if the receiving system does not process both video and audio in a timely manner. I believe my browser/computer is introducing a delay in the audio in some of the circumstances described above.
Whether it is the lack of a synchronizing signal from the server, or an issue with the codec specifications, or a computer/browser issue, I do not know. Occasionally (but very rarely) I see it on other streaming services besides the servers the Church uses.
That will be 2 cents, please. [grin]