I am facing this problem since yesterday and It never happened before.
The Screen is dimmed when I watch a movie or listen to a song in Windows Media Player (Windows 8.1) and once I minimize it or move to another app such a chrome for example, the screen brightens itself automatically.
The problem Does not happen in Classic Media player however...
The problem only occurs when Windows Media Player is in Full Screen mode. I have not installed or modified any settings and I have not installed any new hardware.
under the power options, I have selected High Performance and also I have made some custom settings like never turn off monitor and processor to 100% even on battery...
Another thing, when I right click in Windows Media Player while the screen is dimmed, It brightens again.