Based on this , there is no way to disable the HDMI audio output. (Sadly, there is no 6412 III manual here to help shed light on the subject.) On the other hand, we have long believed that the 990's DVI switching did not pass HDMI audio anyway, which should make it a moot point. Do you have any other audio connections coming from the cable box that might be going to the TV?

It may be a silly thought, but make sure that the cable box is connected to the 990 using coaxial1 and not something else. Also make sure the cable is solidly connected at both ends.

The blue screen sounds like an HDCP handshake hiccup of some sort. Nothing comes to mind right off as far as a solution, but I'll keep thinking on it.