Hi Roger,
I used to have the same problem, I cant \ dont remember exactly what sorted the problem out but I do remember re-installing all drivers an updating them as well as codecs, ( a good multi purpose one is SLD ) available
here
Sorry I could not be more exact, if I remember what it was I will let you know.
ps its not something daft like the mike to close to the speakers, or something else you just changed on your pc set up. How long has it been happening ?