Uninstall, and re-install the newest drivers. Thats the first step
I got this problem about a day after I bought the card in fact, and I fixed it by making sure I never used Entertainment mode ever again, as its as buggy as hell.
Switch over to Audio Creation mode for the cleanest sound, and when playing games, switch to Game Mode.
Entertainment mode is a problem, and can cause problems like you are encountering from time to time.
Easiest way to change it is through the Creative Audio Console, and using the settings there