I've been having this problem for some time now; pretty much since Windows 8 did a recent update several months ago to "8.1". Sound in Movie Maker is hit and miss. I also see quite a few other people here in the forum with similar and related issues with sound in Movie Maker, so it must be something that is fairly widespread.

In my case, often the first time I open a movie file (usually a .mov file, the default export from my main camera, or otherwise an .avi file when I use my little Flip camcorder), the sound in Movie Maker will work. The program goes through the "Movie Maker is making your files editable", or whatever the notification is specifically, and the sound will work.

The next time I open the same file, either I get nothing but loud clicking, or (every so often) no sound at all (as if it is muted). 

The sound on the same clips work perfectly fine in other programs ... both in Picasa (well, the .avi files do, Picasa won't play .mov files anyway) or Media Player.

It is only in Movie Maker, where I really NEED to be able to accurately hear the sound, that it won't work.  It does not matter, also, whether I'm working on the original file, or whether I have saved it as a "project" and tried to reopen it that way. 

I've been several days trying every suggestion I'm finding here on this forum to no avail. I've changed various settings and bit rates, I've done a "repair" through control panel, etc, etc. Nothing seems to help.

Movie Maker used to work just fine on my XP and Windows 7 machines. There is something about this "Essentials" version that is terribly buggy.