Why does over-clocking a CPU cause audio in Diablo 3 cinematics to go out of sync?

I have my I5-2500k OC'd to 4.5 Ghz. Your problem could have more to do with AMD specifically if it is indeed the processor that is the root of the issue. Is your machine a pre-built? I know that many pre-builts are not meant to be overclocked and the Power Supplies supplied are already at a ceiling.

I am assuming that this problem is only happening with Diablo 3. If not, then I would even go so far as to say there is an issue with your motherboard. Try to look up the model number and download all up to date drivers from the manufacturers website. Also, do the same for any discete graphics cards and your processor itself.

If the problems persist, please post back and I can try to suggest something further.