This 'may' be a simple fix....

There are a number of 'channels' in your sound system, as shown inthis image.
It does happen that some programs set certain sliders to minimum when playing some formats. You need to check the settings in the "windows mixer". (I have an icon in my tray that brings it up)

(One program, which plays Real Audio files, ALWAYS sets the volume to zero at the beginning of each song, and I have to remember to manually change it)