After installing ubuntu-restricted-extras on Ubuntu 20.04 LTS Desktop, I can play MP3 but not MP4 in both Firefox 82.0.3 (64 bit) and VLC 3.0.9.2. Ubuntu is running as a guest in VirtualBox on a Windows 10 host.
When I play an MP3 in FF and VLC, there is no issue. Sounds good.
When I try to play an MP4 in VLC, the video is fine but the sound is just quick bursts every few seconds.
Firefox's behavior is even more difficult to understand. If I try to play a local MP4, the first time I hit play it does nothing. If I pause it and hit play again I get both video and audio. The third time of pause/play, nothing. The fourth time, fine. And so on.
I've tried researching this on Google for the past two hours, but the solutions all point to installing ubuntu-restricted-extras, which I have done. I also tried installing ffmpeg. And much of the advice is very old pointing to packages that no longer exist in the repository.
I'm lost and any suggestions are appreciated.