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#11
Resetting the BIOS, assuming that the BIOS is not corrupt and working OK would enable the sound chip. It may be a hassle, but check to see if the CMOS (BIOS) backup battery is accessible. If it is, do the CMOS reset by removing the battery for at least 10 minutes (Intel says 15 minutes). This will force a complete BIOS reset. I've seen cases of the software reset with the F keys not fully reset a BIOS.
I don't remember if this is an Intel or AMD chipset motherboard. If its an Intel, download and reinstall the motherboard chipset drivers (or get them from Intel). If the chipset drivers were corrupted, it can cause devices to not be recognized and your problem, if its not a BIOS problem.