Sound not working after Home Premium installation

I just installed Vista Home Premium edition, after having had Vista Basic.
And once I installed it, the sound stopped working. The computer says there
is no problem, that the speakers are working just fine, but not a sound comes
out of them.
Yes, I have checked for the most obvious things: The speakers are connected,
they are turned on, I've cranked up the volume but with no result. I've
checked for any updates on drivers, I've installed the Windows Updates. I've
tried to do a system restore, but I can only restore it a few hours back, and
the sound didn't work then either.
What should I do?