I have just experienced the same problem and I don’t think that this is a hardware problem at all, as I switched through several output devices and they all had the same sound.
Devices in use were Asus Xonar DX (tested with Headphones as well as 5.1) and nvidia geforce 480 hdmi output to my stereo.
Unfortunately I did not test to change sound quality to see if the problem would go away, I had it set to the highest quality though.
The error occured during a WvW siege on Dreaming Bay.