MP4 audioi

Hi there

I somehow 'programmed' Windows Media Player as my go to video player designating it for MP4.

I want to change this back to the Movie & TV app that I use all of the time
(when I use the M & TV app I see the video, but there is no sound). 

The videos and sound play in WMP but I could find no way to change it back to M & TV.

Thanks for any suggestions...