One suspicion that I find myself having is that the 990's DVI switching uses dual-link connectors and that this actually does allow HDMI audio to pass. We'd believed that the way it was wired simply abandoned the audio data of an HDMI cable, but it's possible that an HDMI to dual-link DVI cable at each end would allow the HDMI audio to travel successfully through the 990's DVI switching (assuming both HDMI-to-DVI cables were wired the right way).

All of that aside, the problem remains that we need to get coaxial digital audio from the Motorola to the 990. If there's still no way to disable the HDMI audio output, you should at least make sure that the digital audio output is set to "bitstream" or "raw" to insure that the SPDIF (coaxial/optical) output is getting the proper signal (otherwise you will never get Dolby Digital audio). It might even be necessary to find a different HDMI-to-DVI cable (perhaps one with a single-link DVI connector) to see if that breaks the HDMI audio signal in case that's causing the Motorola to disable the SPDIF output for some silly reason.