Thanks to both of you (since I'm not sure whose advice did the trick). But the trick has been done . . . and I now have sound again. Panic over! The man at the computer repair shop didn't charge me for pointing out that the F1 to F4 buttons have - on my keyboard, at least - pictures of speakers in order to increase, decrease, mute and unmute. So, it had been working all along . . . but I'd inadvertently muted it. Thanks again!