If you are browsing an edX course on Linux (Ubuntu, openSUSE, Debian, etc.) and have trouble watching videos, this could be because edX uses some video codecs that might not be available with the installation of your OS.
In order to solve this, please follow the following instructions to download and install codec packages:
Ubuntu: https://itsfoss.com/install-media-codecs-ubuntu/
openSUSE: https://en.opensuse.org/SDB:Installing_codecs_from_Packman_repositories
Debian: https://linuxhint.com/install_multimedia_codecs_debian_10