Why do my videos on my PC play in 'slow motion'

I had the same issue after an update to both my nvidia video card and a windows 10 update. Everything was working fine and then the video slowed for both windows media player and the movies & tv app. VLC was fine. After searching and searching.. I found this question and answers and wanted to confirm it is a sound issue. I have the sound coming through the hdmi to my tv monitor. And for some reason, the default sound device had switched to a different output device. I switched the default device back to what it was and the video started playing normally again. One of the updates must have messed up my default sound device. So anyone else who stumbles on this problem, check your sound settings in the control panel. keywords: windows media player, slow motion, playing slow, movies and tv app

So, by accident I have found the problem. I use wireless headphones instead of speakers for my PC. When I unplug the audio cable from the PC, videos play slow, but as soon as I plug it back in, they play normally again.

No idea why this is the case, but it has fixed my problem.