Please try all the fixes, but...
Well, things are not good. Do you have the dvd/cd still working? Mine is not working along with the same sound problem. Some days they work, when they work they work in the meanwhile, when they didn't they didn't together.
I tried all the fixes, none worked. I have reinstalled, sound and chip drivers, then BIOS, and ... Vista after formatting,...useless. It is like the sound and dvd hardware are missing from the system, they are not listed at all in the device manager list.
Unfortunately I think it is a overheating problem, did your laptop shut down by itself because of overheating? Mine did, and probably the chips soldiers or the wires are broken. Still a strange software conflict can do the same job, I HOPE is this, but I have run out of ideas.
If anybody fixed this(not the easy things, like drivers, bla bla) please let us know...