Voice on and off in Movie Maker

The Movie Maker has always worked. Just today, I found the voice gone (mute) at time--in a newly created wlmp file or in a previously created wlmp file which had worked properly earlier. I played around with this and found the following: that the voice comes on and off randomly. I would click a wlmp file and click "Preview Full Screen" to watch. Voice may be on; it may be off. When it is off, I would close the Movie Maker, then on, then off, and on, and off... Then, after 4-5 on/off, the voice comes around. No problem in the past. I recently upgraded from Windows 8 to 8.1. Thank you!