I recently purchased a surround system so my challenge was to get the surround information from my computer to the receiver. This is what I found worked for with my SB live 5.1 sound card using the spdif connection.
First go here and download and install ac3 filter and spdifer.
http://www.ac3filter.net/
Leave the default setting in ac3 filter except for selecting the output format to PLII mixdown, also leave all settings default on spdifer. Now you can play movies with ac3 tracks in Windows Media Player, or Power DVD. The receiver will still tell you that it's processing in PLII mode but the sound should be quite discreet, and it will actually match the way it was encoded because it was decoded by the filter, and re-encoded to be passed over the limited bandwidth spdif that most computer sound cards use.
I hope this helps somebody out.
