I've had less trouble with CMedia onboard than most describe having with their Creative cards. It depends more on what type of onboard sound you have I suspect. I never have to turn down acceleration like some Creative folks seem to do for many games.

Check your system for spyware and trojans, you may have something executing in the background and bogging your system down. Have you downloaded any video codecs? They might be interfering with the sound in some fashion--usually the problem is only in the game videos. Use DXdiag to make sure your sound is configured properly for DirectX.