Darn!
I'm not a sound guru or master, so I don''t have a lot of advice to offer, but there are still a couple things that you can try.
You didn't state, are your headphones USB or mini-plug?
If they're USB, try a couple diff. USB ports. If they still don't work, open Device Manager and see if there are any question marks or warnings for any Human Interface Device (s) or USB. (I know, you've probably already done this-if so just disregard)
You probably have already looked into this but: Restart the PC and go into the BIOS. Find the settings for sound and make certain your on-board RealTek sound is selected. There MAY NOT be anything that says RealTek or on-board sound.
My board (ASUS Sabertooth) only asks if I want to use AC97 or HiDef sound. Choose one and boot, see if anything changes.
Make certain that in the Windows sound mixer (speaker icon in bottom tray) the speakers aren't muted. (I know, you tried it, but every once in awhile we humans make a mistake and don't realize it and shut off our sound. Done it a few times. Windows Updates are good at doing it, too).
Last, at least for now, try the headphones on another system; any system - stereo, t.v., game console, etc. - to make sure they are still working
Good luck. Last person I helped we narrowed it down to a defective sound chip on the mobo, caused by a power outage. His fix was simple; a cheap sound card.