Encountered the same problem two days ago. Mine is Compaq Presario f700, Vista 32-bit. Tried most of the things proposed here and elswhere (updating drivers, BIOS, etc.). Did not work. Saw on one forum an advice to put the laptop in a hybernate mode and then turn it on again. As a last resort, did just that, additionally removing the battery and then inserting it back. And it helped! Sound is back - physically and virtually (reappeared in Device Manager and other services). Hope, it might help you, too.
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