I have what is hopefully an easily solved problem with a Shuttle PC w/ onboard Realtek AC97 in Windows XP SP2. It is connected to a Panasonic 5.1 receiver via SPDIF (optical, not coaxial). The Panasonic receiver displays signal information on its HUD, including whether it's receiving Dolby 5.1, Dolby Stereo, DTS, or PCM; and for which speakers it's receiving a signal.

Using the ATi DVD player, I can play DVDs back in surround sound if I tell it to pass audio through to SPDIF ... so far so good.

But if I set windows for 5.1 speakers, and the Realtek drivers to 5.1 speakers, the system still outputs stereo sound. The receiver display indicates that it's receiving 2 speaker signals in PCM mode. When I test individual speakers in the Realtek tool, only the left and the right front speakers play any sound. It's as though it's processing sound for 5.1 speakers and then only sending the front left and front right channels to the receiver (and just dropping the other 4 into oblivion).

Resolved: It turns out my test file was "bad" (or rather, I didn't know that WMA surround sound files can't be played in surround over a digital connection, only analog).

I'm largely satisfied that things are working correctly and I just didn't understand how limited WMA playback was.