Hmm... I have that same problem on my onboard sound that came with my motherboard too when i first got it, so I just thought since it uses CPU power, that that was the cause. It does use CPU time though, and that would explain that why that lags, your computer does.
But I have a Sound Blaster Live card that works seperatly to the CPU so that there is no lags.
I would buy a card that will plug into a free PCI port on your motherboard and replace the onboard sound.
You can get Creative Soundblaster Live cards rather cheap if your not all that interested in surround sound, and even the standard Live card will support 4.1 speakers like I have. That'll cost about £30 now.
If you wanted more, you could go for ones for not much more that would support 5.1 and so forth for movies.
So worth checking out some
At least your computer wouldn't lag anymore with sound.