Problem with playback in Movie Maker

I am working on a project in Movie Maker and I have a small clip of a song that I have copied and pasted several times over. I copied and pasted about 4 times, then tried to play the project, and it didn't work. I clicked on the play button on the preview window, and absolutely nothing happened. I tried restarting the computer, and then copying all the video and sound into a new project; it worked with the video but not the sound, so I had to do that from scratch. When it came round to copying this clip again, the same problem happened. The writing on the preview pane is grey, like it is unselected or something, but nothing I do has any effect whatsoever.
Many thanks

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