Have you tried changing a few settings in the Xonar control panel? Might be an idea to mess around with it unless reported by other players as a common issue?
Wow this is a looooooong game!
Currently at: Section c1a4b: GETTING TO THE ROCKET FIRING AREA
Used a walkthrough a little as I got a stuck and was running around in a circle.... Anyway back on track now.....
Mainly been playing lunchtimes at work and the odd occasions at home.