Recently I helped someone out as they were complaining that iTunes had stopped playing music videos. The song would play in the background but no video was being displayed.
It's a simple fix as he had iTunes to play out of multiple speakers, as he has an airport express hooked up to his home stereo system.
In iTunes in the bottom right hand section on the screen if you have mutiple speakers available you have icons like the one below.
If you have more than one selected then it looks like
If you click on the Blue icon it will show you what speakers you have selected
So if you set it back to just computer then your music videos will play.
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