*BUMP* I've been trouble shooting this same problem here in the future! In the year 2014. I'm writing this so someone can find a solution in the year 2018.
I took my computer offline and cleaned the internals from dust and replaced the second CPU fan which was keeping noise. After putting it back, only front panel audio out worked. I tried all sorts of spells on this (uninstalled Razer surround software, rolled windows back to a recovery state which I knew should work) but none of them worked, until I managed to find Windows 8 drivers for my Realtek audio (MB is Asus M4A89GTD PRO/USB3 btw just so that search engines might find this) which came with a software that started to detect what I'm plugging in and out of audio jacks.
Finally for the 20th time I decided to try all the 6 jack one more time in the back and for some reason the red jack now made Realtek software ask whether I'm plugging speakers or headphones. For some reason I answered headphones instead of speakers - and it worked. Now I have sound from back panel again.