Sound problems from Gigabyte MA790XT-UD4P and Win 7

My setup is a set of Creative DTT3500s connected to the S/PDIF output of the MA790XT. The problem I'm having is getting sound out when using the Win 7 driver from the Gigabyte website. For those of you that know the DTT3500s, I have the following lights on:

Fourpoint/5.1 DIN
Coaxial
Line In
Digital In

At the moment, I'm using the driver Windows installs itself. This gets me upmixed sound to all speakers from, for example, iTunes. Half Life 2 refuses to recognise more than 2 speakers. If I install the Gigabyte driver, I can't get any sound at all. In either case, the MA790XT doesn't appear to output a Dolby Digital signal, as the light doesn't come on on the amp. In contrast, my four year old A7N8X-E did this flawlessly.

The optical out port on the 790XT gives me the same results.

Is this a driver issue? The mobo? The speakers? Software? Me?

I'm looking at discreet soundcards in an effort to sort this out, but I'm surprised the mobo doesn't output a Dolby signal.

OK, I've made some progress here. I've got the Realtek driver installed (claims to be a Vista 64 driver, but seems to work). Also found the Dolby Home Theater software on the driver disk, which has given me my Dolby Digital output :D

Half Life 2 still refuses to recognise 5.1 speakers though, is this a recognised problem?