When I start to play podcasts or films I have only sound.
You may want to try a couple of troubleshooting steps located in this article to address this issue:
Specifically, attempt these steps to see if they make any difference to the playback of these videos:
Reset the system
Reset the resolution
Start by resetting the Mac's parameter RAM. If the display does not come up, was previously set to an unsupported resolution, and still results in no video:
- Start up in Safe Mode.
- From the Apple () menu, choose System Preferences.
- Choose Displays from the View menu to open the preferences pane.
- Select any resolution and refresh rate that your display supports.
- Restart your computer.
Thanks for using the Apple Support Communities. Have a good one!