I too have the OEM version of PDVD 7.3 from my Sony BD drive purchase. I thought the same thing until I did the following:
1. I updated the PDVD software. It was a large update ~70 MB and it changed some of the audio settings. You can access the update by going to system settings and I think choosing the upgrade feature. There's an option to upgrade to PDVD8 (costs $) or download a critical update for your software (free)
2. In the configuration menu, under the Audio tab, I chose the SPDIF output and the option to play the direct audio stream (no downmixing)
3. When I put in a BD movie, I make sure that I go to the audio configuration menu and select the appropriate 5.1 digital output stream. I think most movies have this option, though some may not.
I think the main problem is PDVD7 doesn't have the option to output 5.1 analog for BD movies, nor does it downsample 7.1 streams to 5.1. Hopefully the above will work for you, but note that I'm not at my HTPC, so I'm guessing at what the options are actually called. Also, if the movie doesn't offer a 5.1 digital soundtrack then you may be out of luck.
Hope this helps.