Remove the graphics card, re-boot into BIOS, set the video to on-board chipset and boot. Test sound in GP4, the on-board video chip should display GP4. If the result is the same, then the fault is with the on-board sound chipset. You should be able to get a driver, download and run Speccy to get info on the sound chipset. If you have sound after removing the graphics card, then replace the card or try a different PCI-E slot if you have one, the sound and the card may be trying to use the same address.
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www.gp4central.com] <----GP4 Central