Comment 12 for bug 708957

@Julian: Thank you very much, your comment most probably contains the solution for this bug.

I just re-tested all combinations:

kernel 2.6.32-27 without alsa-backports: HDMI sound does NOT work.
kernel 2.6.32-27 with alsa backports: HDMI sound DOES work.
kernel 2.6.32-33 without alsa-backports: HDMI sound does NOT work.
kernel 2.6.32-33 with alsa backports: HDMI sound DOES work.
(The hack described in my previous post was not needed.)

> If the 2.6.32-28 kernel update didn't come with the corresponding linux-backports-modules-alsa-$version package, a change in audio behaviour sounds reasonable.

As far as I can see: That's it! According to my Synaptic chronics, on 2011-01-27 01:15 indeed only linux-image-* but not linux-backports-modules-alsa-* were updated to 2.6.32-28. After submitting the bug report above, I reverted to kernel 2.6.32-27 and inhibited any later kernel update by uninstalling all virtual packages depending on the most recent kernel version number. So linux-backports-modules-alsa-2.6.32-28 (which must have been released later) has never been installed on my system.

Solution for my problem: Defer all future kernel updates until the corresponding alsa backport update is also available and always install them together.

Thanks!