VLC Player opens the file and plays it, but there is no sound. YouTube videos do the same: they look normal but there is no sound.
There could be up to 3 controls for volume: a wheel adjuster on your computer, via the keyboard (Shift-F8 or similar, and a mute key), and finally the volume slider on the program itself. Are all three controls open? If you are using earphones, is the jack active? Have you checked that the sound settings in the control panel is hunky-dory?
As far as I can work out, these controls are all correctly set. Put a different way, the problem started up without me having changed any settings or controls.
I have tried out all the stupid things I could think of, e.g. trying out different head phones, trying out external speakers, etc. etc. Nothing works. Media files open and play, but instead of their normal sound I just get noise - the sound now is something between a helicopter and machine gun stutter. In fact, unless I mute the computers sound, this sound is up non-stop, whether I am playing a media file or not. Same thing happens on YouTube.