Could you try playing some non-protected .wav file in WMP?
Anything that doesn't require actual passthrough (but would use PCM output), such as MP3 or WMA works fine in WMP even with the SPDIF device selected. Only DVD has a problem. The Creative driver is not signed, but the Realtek driver is, but as I said the problem
occurs on both.
Plus, if it was that, why would Media Center be able to do it but not WMP?
One thing that does have to do with copyright protection is that both WMP and Media Center refuse to let me use my main DVD drive (which is RPC-1, i.e. region-free) so I have to use my DVD writer (which is RPC-2). But in this case, both tell me that there is
a problem with copyright protection preventing playback.