Usually the FFMPEG options are grayed out because like most people you don’t have FFMPEG installed on your server. If you know that FFMPEG is installed on your server, go to the FFMPEG Settings tab in the “Video Embed & Thumbnail Generator” settings page and enter the full path to the application folder on your server. By default it’s /usr/local/bin
but it’s sometimes installed in usr/bin
or it could be any number of places. If the plugin is able to execute FFMPEG from that folder then the settings will be enabled.
I’d still be happy to help you troubleshoot your playback problems, which are generally unrelated to FFMPEG.