Problem: HMDI audio distorted (MSI R6850)

Hello all,
(I thought I already posted the question here, but a search came up empty. So here we go again...)

I have a weird problem with the HDMI sound on my MSI R6850 (1GB). It's been there since I bought it about half a year ago, though five updates of the drivers (though I'm not sure how often that included updates of the specific HDMI audio driver).

My problem is that the output of the HDMI sound is distorted. It sounds tinny, sometimes there is a humming noise whenever a sound plays.

What I do then is go to the Control Panel and select "Manage audio devices". The first one (though not my default one as my primary sound output is the onboard one - I use the HDMI connection as a mediacenter output to my TV) is "Realtek HDMI Output", of which I select properties. In the "Advanced" tab, I can select a default format for the output, which I then change to one of the four:

16bit, 44100 Hz (CD Quality)
16bit, 48000Hz (DVD quality)
24bit, 44100 Hz (Studio quality)
24bit, 48000 Hz (Studio quality)

and click on the "Apply" and "Test" buttons. I keep changing and testing the format until the distortions stop. Sometimes, I even have go through all four more than once.

Once the distortions have stopped, they don't come back until I reboot the computer. But as soon as I do that, I have to go through those motions all over again. The "correct" setting for the audio format keeps on changing.

I have both a (Samsung) TV and an (Onkyo) audio receiver connected (via a 4x2 HDMI switch) to the graphics card. None of the other HDMI inputs (DVD player, PS3, cable set-top box) have these audio problems, only the computer. The sound is distorted on both the TV and the audio receiver in the same way, so I presume it's a problem with the card and/or its driver, not the TV or the receiver.

Has anybody else ever had that problem? Do you have any idea where it might come from or what I might do to get rid of it?

Greetings, Michael.