While I'm not being knowledgeable in Hebrew syntax, my understanding would be that a maiden known to the audience conceived
(in the old fashioned way, I reckon), and the child served as the sign against Ephraim and Damascus. I see no reason to believe the almah had to be Hezekiah's mother or Isaiah's wife.
I don't think anyone in this thread has mentioned it (not that I agree), but I have heard some keep the meaning of virgin in Isaiah 7:14 by reckoning this woman referenced was a virgin at the time Isaiah prophesied. She would then marry and conceive in the old fashioned way.
I have been told by a rabbi (And remember, he does not recognize Jesus as Messiah or God) that if an "almah" is/was single, her virginity is a gimme. If it's known a single woman's not a virgin, she can't be an almah.