As I understand you do not have MUTE button on netbook and Window 7 mute option does not work
You can reinstall the Audio driver so in Windows 7 the mute option works properly.
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I have been away for a couple of days and haven't been able to take further steps, but I just found your advice waiting and the mute facility is now restored.
The machine isn't a new one but has been passed on to me by a relative who recently got a new laptop, and this fault was existing when they had it so it is nice to have been able to fix it. Thank you again.