just updated my muse, and for some reason and can't read the MP4 video files that worked in the previous version
That's strange. Are you using the widget in Muse or are you using a third-party widget? If you are using a third-party widget, contact the developer to update the widget to be compatible with the latest version of Adobe Muse.
Until your issue is resolved, you can install and use this version of Adobe Muse Install Adobe Muse version 2017.0.4