Comment 14 for bug 589316

I also have exactly the same card and similar problem. If I install PulseAudio Volume Control,
 I am able to see the individual channels in the Output devices tab and adjust them (Front L, Front R, Centre, Rear L, Rear R, and Subwoofer)
Strangely it seems the lower these levels are the worse the sound quality seems (A tinny noise that sounds like a poorly encoded MP3 or blown speakers)

If all the channels are set to max the noise is gone, interestingly if I leave all levels at maximum, then just lower the subwoofer channel the noise comes back even if its only reduced by 1%. On the other hand if I max it up again then select another channel say Front L for example I can reduce this by 90% to 10% and still no noise yet as soon as it or any other channel hits 8% the horrid noise is back.

If I then set both Front L and Rear Left to 0% I can hear the right hand speakers perfectly and vice versa. So it seems to be caused by splitting the front and rear channels and having them at disparate volumes.

Very Strange

Regards Richard