Nero

I have a 32 inch lcd tv. It's a great set and displays HD beautifully. I picked up an upconverting Toshiba DVD player that is HDMI. So I got a HDMI --> DVI connector. My problem is that regardless of what the DVD player is set on, I cannot get true DD 5.1 out of it with an optical cable, it's very weird. For some reason, my receiver only picks the signal up as Dolby Pro Logic 2. However, if I go with component cables into the TV, and run the optical cable, it comes up with DD 5.1 or DTS perfectly fine. Anyone know what is causing this and how I can fix it?