After a lot of further trial and error, I discovered that the cable connected to the speakers had come off loose (disconnected). You see, my computer is in a small corner of our spare bedroom in a tiny maisonette and therefore my cables are not always easy to reach as one has to stoop down to the floor and, more or less, work blindly to check the cables in a hard-to-reach corner.
Thks again for your help, particularly yr patience with me.
GUZEPPI from the Middle Sea.