Disaster! The wretched laptop has recovered its ability to make sound without headphones.
I took your advice and repeated the tests, and to begin with the results were as before:
sound fine with headphones
sound comes on briefly without headphones, making you think all is well, and peters out and goes altogether.
Then there was a time when the sound (without headphones) carried on and carried on, and now you wouldn't think there had been a problem at all.
I was always going to go to PC World tomorrow, Thursday, when we are going to Hereford anyway, and today I'll repeat the tests again once or twice.
I'm inclined to take the laptop into the store, with the headphones, and with written notes about what has happened, plus my proof of purchase, and see what they say. Can you think of anything else I can say or do to strengthen my case?
I suppose the one good thing is that this miraculous cure happened before I got there - it would have been terribly embarrassing if I'd been taken by surprise when I went to demonstrate the problem!