What driver versions are you using for the Realtek and the X-Fi?
Are these 32-bit or 64-bit machines?
This is the RC1 build, I assume?
Sorry, I should have mentioned. Yes, it is RC1, and it's x64 (although the same thing happened on 5536 x86).
I'm using the latest Vista beta drivers for both the Realtek and the X-Fi. I just discovered I actually wasn't using the latest drivers for the Realtek, so I installed those, and now Realtek's "Digital output" endput has the same type of options as the X-Fi's
"Spdif-output" (although it supports considerably less sample rates).
Using this now functional Realtek option, Media Center performs much better; there's still the occasional glitch but it doesn't lapse into silence or continuous glitches anymore. So it seems that part at least is to blame on the X-Fi driver.
WMP, PowerDVD and Media Player Classic show no change. No sound whatsoever when using SPDIF out. It's also interesting to note that they all have regular playback stutters (about every 5 seconds or so) when configured to use SPDIF, which don't occur using normal