No Sound from Audio-Out, but sound from USB Headset

On New Years, one of my hard drives failed. It contained my Operating System (Win 7/64bit) but not much else... so nothing of value was really lost. However since then, I've had a week of tweaking, tuning, problem-solving, and returning to the basics of building a computer just to get things functioning properly again. One of the strangest issues I had after I had installed a fresh copy of Windows, was that my otherwise normally-functional audio device (inbuilt soundcard on EVGA 780i FTW) was not working... and not just not working, but refusing to show up in Windows at all... I spent a few hours with different drivers and configurations and numerous restarts, and finally cultivating in yet another fresh Windows Install before I worked out the problem... and then I felt stupid.

If you have a Modern...ish (2004 onwards) computer, you'll probably have a case and a motherboard which will have two different audio cables, called HD Audio, and AC'97, these cables allow you to connect the front panel of your case to your sound card, allowing you to use the front I/O panel audio, which is usually just an headphone jack. Like this:

source: http://ask.creative.com/wwimages/audio_int/xfi/xfi_extremeaudio_frontpanel2a.jpg

And of course... this:

source: http://cdnsupport.gateway.com/s/Cases/GERSHWIN/FRNTPNL_AUDIO_RCA_BAY.jpg

Should be pretty self-explanatory. Anyway, my sound card was not working, but, curiously, my USB headset was... this lead me to falsely believe my sound was perfectly functional, because I plugged that in before my speakers. Turns out I was wrong, most USB headsets actually contain a tiny driver and sound hardware to produce noise, they usually don't rely on Windows. When I plugged my speakers in and there was no sound (and Windows refused to even acknowledge anything was plugged in), I figured something was up with the driver, no that was fine (actually it became corrupted but there's neither here or there) and after banging my head against a wall trying to work out what was up, I ended up just deciding to look up what AC'97 stood for, which, after a few page hops, lead me to this sentence on Wikipedia:

"The different signal assignments can cause trouble when AC'97 front panel dongles are used with HDA motherboards and vice versa. A loud audio passage may make the HDA motherboard with AC'97 dongle believe that headphones and microphones being plugged and unplugged hundreds of times per second. An AC'97 motherboard with an HDA dongle will route the 5 V audio supply (silence) to the speakers instead of the desired audio."

Welp, I'm an idiot. So I unplugged the AC'97 cable, and then sound returned to normal. Message of the story is: Don't plug HD Audio and AC'97 in together at the same time if you value your sound.