I'm having this weird audio problem.

Audio on my pc works fine however often when playing the Likes of Civilization IV and NWN 2, i completely lose sound. It happens 80% of the time, sometimes it doesnt happen at all. Sometimes it happens after 2 mins of playing.
As soon as I quit the game, sound is back to normal... 100%
When playing PES6, it happens less often (approx 5% of the time)

Watching movies or playing music also works fine.

Anyone experience similar problems or know where i should start looking to resolve the issue.