The reason I made the suggestion above is that I ran into a similar problem that was related to another piece of software, TotalRecorder. Their FAQ, at http://www.highcriteria.com/faq_tr.htm suggests that the problem is related to the DirectSound driver. So, you might play around with that a little, perhaps by running the DirectX diagnostic tool (from the Start menu, choose Run and then enter dxdiag).
I also would imagine that googling garbled audio itunes quicktime should point you to related information.