The only thing that comes to mind, is the ability WMP has to slow down and speed up the speeds of audio/video, so when viewing the video, right click and check out the Enhancements; you may have accidentally changed these, or perhaps a keyboard shortcut did it. TBH, though... I strongly
recommend you use SMPlayer as it is substantially superior to WMP; as much as I love WMP, it doesn't compare, when it comes to videos.
SMPlayer - Downloads