I have Windows 8.1 Pro.  I made a football highlight video with a bunch of 3-8 second video clips in .MOV format. Used slow-mo in some of them and added a song and a title slide.  I saved it as a movie maker video (wlmp file) and played it at least 5-7 times and it worked just fine.

A few hours later was going to play it again.  It played, but had no sound.  I checked all the volume settings and all was fine.  I paused and restarted the video several times and eventually just played it all the way through.  Still no sound, but all videos played.  I close Movie Maker and open it back up and now I have sound.  BUT, now I have a black screen in the preview window on all videos except the title and the first video.

I search through this forum and see I am not the only one with this problem!! Wow!!

So I find that I need to check my video card (it's fine according to my computer).

I see I need to do something with codec, but I'm not sure how to do this.

I also see that I can convert the file to .wmv format and that will solve a bunch of issues.

So I do this with one of the videos and start a new project in Movie Maker.  The video plays and I edit it.  I set my start and end point, then I copied and pasted a part of it and put it at 0.5 speed.  I play it probably 5 times and it works. 

I convert another video and add it to project.  I put in a title slide.

I go to play again.... And same problem.  Title slide and first video plays but the rest do not.  There is a black screen. 

The 1st, 2nd and 3rd videos are all from the same file, same video, just clipped in a couple of different places and one of them put in slo-mo.

What is this problem!?!?!?