I have the same problem you have. I upgraded from PVD7 Ultra to PVD Ultra 8 and found a work around. It remembers the SPDIF settings when start the Blu-ray in PVD8, but when it goes to the main feature it defaults back to 2 speaker. So, to get SPDIF working again, pause the movie while in the main feature, change the audio back to SPDIF (you can't do that in PVD7 Ultra - I tried numerous times), then unpause the movie. If you have problems with the audio not coming through, skip a chapter and watch the audio come in its digital form. Of course, then you can skip the chapter back to start the movie.
All in all, I would like to see a permanent fix to this problem instead of having to do this everytime I watch a movie.