Odd one - in the windows sound settings (properties), do you have it set to the highest you possibly could? Say 24 bit 96000hz or Higher? If so, try it on 24bit 44100 or lower. see if that helps much. I doubt you'll notice much difference in sound quality in games and movies, as it's usually all compressed anyway. Try this before the rest below.
There should be a PCI latency/timing setting somewhere in your BIOS, if set to 64, knock it to 32, or vice versa. I wouldn't set it to anything higher than 64 though (higher = worse, but can improve stability in some cases.)
I'd also check your mainboard manual to see what devices use what pci-lanes and IRQ settings - if your GPU shares lanes with a usb controller for instance, if you have free ports, move any devices from that usb controller and disable it. Same for your sound - either on board or PCI-e.
Also on your board, is there an option to enable DMI 3.0? if so, enable that. give it a whirl.
disable usb power saving for all your usb controllers in windows device manager.
if you have these options on your NIC, disable adaptive inter-frame spaing, raise the send/receive buffers to maximum. Disable interupt moderation also.
Good luck