I believe I have the solution to your problem. You're the only person that I've found on the internet with the exact same problem I'm having. Everything you described with your sound on your first post has happened to me.
I recently built my first system. I have the EVGA x58 SLI, running Windows 7 Release Candidate x64 OS and using the onboard audio with the Z-5500 5.1 surround speaker system.
With me, the buzzing starts at random times, usually when I'm playing music on Windows Media Player and even then it only happens after a while of playing different songs and switching between songs. Rebooting, reinstalling the audio drivers from EVGA, installing the drivers from the RealTek website, and running it without the RealTek high definition audio drivers, all only fix it temporarily. The buzzing eventually starts up again.
EVGA customer support was clueless to the cause.
After doing restores, and messing with all kinds of settings I found that removing the "speaker fill" option either from the "Realtek HD Audio Manager" or by going through the "Speakers Properties" and into the "Enhancements" tab, fixes the problem. "Speaker Fill" creates a virtual environment for playback of stereo audio sources on surround speaker systems. I'm guessing that the motherboard starts trippin' out after a while of making my stereo songs play through my 5.1 surround system as if they were songs in surround.
As a tip, make sure the buzzing is not present when you go ahead and turn the "Stereo Fill" off. Then reboot the system just to make sure.
So far, I've been playing songs, games, and 5.1 Blue Ray movies for a couple of days now, yet the loud buzzing has not come back. Fortunately, my Z-5500 setup can do it's on "speaker fill" effect when I set my settings to "Direct Input" and on "PLII Music" effect. It receives the music at 96000 - 24, which is the highest quality that the speakers can handle.
Tell me if this works for you. It'll feel good to know I wasn't the only one with this problem. EVGA has to do something about this problem.
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