Hard to say. Problem with asking is that there's no simple yes or no answer. Some people may notice a difference, some may not. Onboard audio is good enough now on better quality boards for the difference to be small enough for some not to notice.
As you've got both, I suppose it depends on whether the U7 is something you've owned for a while, or new and still in packaging. If it's used, then trying it and comparing for yourself is a lot easier than it would be if it were an internal sound card. If it's new in packaging and you don't know whether it's worth opening and trying it, then I'd probably just stick with the onboard audio.
Either way; unless you're using headphone as well, then you likely won't need what the features the U7 has to offer.
