If you can’t view a video in PowerPoint Online, there’s most likely an issue with the type of video. If the video is a Flash or WMV file, you’ll need a player to see it. However, if you’re using a tablet or other device that can’t run the player, you’ll have to open the presentation in PowerPoint (use the Open in PowerPoint command in PowerPoint Online).
Recent videos are likely to be in a format that plays directly in your browser, as long as your browser is up to date and supports this feature (known as HTML5). So another way to get the video to play is to go to your browser’s website and get the most recent update. If the video still won’t play, your fallback is to open the presentation in PowerPoint.
You can play video and audio in Reading View and Slide Show View if you have Silverlight installed. You’ll need Flash for videos that are linked from a website. Keep an eye on the file size, because PowerPoint Online won’t insert large media files. For embedded media, the size limit can be from 50-100MB, and for WAV files the limit is 100KB.
Note: If you use a desktop computer running Windows 8, run Internet Explorer from the desktop (rather than in immersive mode) to use Silverlight or Flash Player. Also: these players can’t be installed on tablets or touch-screen mobile devices; try opening the presentation in PowerPoint to play the video.
When you are creating presentations
When you add a video to your presentation in PowerPoint 2013, make it easy for people to play in PowerPoint Online. Embed an MP4 file, and then go to File > Optimize Media Compatibility. This command puts your video in a format that plays directly in browsers that support HTML5.