OK...it gets even weirder...
The problem only occurs using headphones (yes, any kind). Music, yes; everything else, no.
Unplug the headphones, and you have all the sound. Plug them back in, it's all good.
Must be some sound driver problem, but damned if I know. I've been using Macs since the lady threw the hammer at the Big Brother screen back in 1984, and I've never seen anything as voodoo messed-up as this...but then it is the BLACK MacBook
Anyway, at least me and my love are reunited for now. Any ideas are still welcome...
Stephen