Strangely enough, if I play a movie (say, from a DVD) with 5.1 audio on Windows Media Player it outputs all 6 channels to the home theater. I think we can safely assume that my hardware does supports surround sound, so the problem must be caused by faulty software (which I think has to do with the Windows sound system).
I guess it works on WMP because of some specific configuration of the player that overrides the system audio output.
The problem is, I want not only to watch movies but also to play games and use sound editing software with surround sound, and they fail to detect my surround system if the OS wrongly specifies my output as STEREO.
As for the other problem, let me illustrate what goes on
This is my system working as it should:
Then I turn my TV off/on (it's instantaneous, notice the difference of 1 minute in the clock) and my HDMI Output gets permanently disabled until I go to the logon screen or reboot.
(on a side note: the default "Digital Audio (HDMI)" never worked for me, so I just disabled it)
I don't use speakers, there's nothing connected to the mobo 3.5mm jack. My primary and only sound device is my TV through a HDMI cable.