Only the headphones? Meaning external and the internal speaker (if any) are OK? First make sure you are not muting the output in Audio options. Second, test the headphones in another system. If they work there, try a known good headphone in your computer. If that also fails to work, you are looking at a bad jack. If the headphones don't work in the second computer, but the other headphones work in your computer, you can probably deduce where the issue is then I'd think :-)
Oh, one more thing, are you using the front headphone jacks? If you are, they are probably not wired up to the new card, you need to swap the connections. Reading the post above about disabling the onboard sound brought this to mind.