Besides what birdy already pointed out....!!!

Did you try to open the file using the built-in player of LW??
If so, check whether the player is enabled....(go LW>Tools>Options>Player).
or
it might be a codecs issue....
You could try to use a different media player such as the free VLC player with built-in codecs which can play almost all media files without problems. You can download it from here:
http://www.videolan.org/vlc/
As a side note, to preview files with it you'll have to open the files directly from the Incomplete folder and not via the LW preview function.

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