There exist some video converters that are capable to convert Matroska WEBM format to MP4 video. Usually webm to mp4 conversion is done because MPEG-4 format is far more supported. Nevertheless, most media players nowadays should be able to playback of .webm files just fine.
Open WEBM file Open MP4 fileA multimedia transcoder for Linux
One of the many multimedia converter programs for Windows
Easy to use video converter for Mac
A free multimedia converter in version for Mac
Universal All Purpose Free Media, Audio & Video Converter Player Recorder
A desktop transcoder app for Windows