I have connected by HD set top box to my Sony 5.1 Home Theatre System. On HD channels the volume drops. The volume and effect is good on non-HD channels.
I've experienced this as well. I don't believe it is your equipment, but an issue with the cable/satellite company or the HD content itself. I'm a Comcast subscriber using an HTPC and an Onkyo HT-S3200 HTiB.