All is solved
I have found the problem and solved it.
My mother board is from" Gygabyte Technology". It has a sound card built in. It also comes with a programme for "Sound Affects".
Hit the icon for this programme and up comes what looks like an organ keyboard with knobs and whistles. You can change the sound to various environments like under water or in a bathroom or in an arena etc. There is also a karaoke set of buttons, giving effects marked from -4 up through 0 to +4. I have never, as far as I know used this programme, but it was set to underwater and the karaoki was at +4. How come? I have no idea.
I set the environment to none and the karaoki to 0 and hey presto, everything is back to normal.
What I cannot fathom is, why would anyone want this programme?
stony
ps. Thanks again for all the suggestions. If I get any more real problems, I shall return for help:D