The easiest way is to play WebM files through a third-party WebM player such as VLC player and KMPlayer, etc. VLC player is a free and open player that can play all the mainstream video and audio formats without additional codecs required. If you don't have it, you can download it from the official website. It is very easy to play WebM file in VLC directly.
Now I have told you four ways to solve the problem that WebM files can't be played in Windows Media Player. You can choose the way you like to solve your problem. If the article is useful to you, please share it with others.