I believe both new dvd formats use some DRM to make it difficult to copy. This DRM only lets 2 channel output go unless you are using HDMI to the AV receiver. In my opinion, this just sucks. I know very new video cards can do HDMI out.
If this happens on regular DVDs, then your software player may not be set to 5.1. Some of the OEM versions of players only do 2 channel. Try digital out from the sound card to your amp.