If you can’t view a video in PowerPoint Web App, 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 the web app).
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.
If the video is a Windows Media or Flash video (.wmv or .flv file), you can play it after you install Silverlight (to play Windows Media) or Flash Player (to play Flash).
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 Web App. 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.