Long story short, no power & no cable for 8 days, i had need of reconnecting speakers to my TV in order to use an air antenna.
When i was able to return to using the cable and 990 i noticed the same audio track was playing both through the tv and the in wall speakers. Seemingly the 990 is passing the digital audio unaltered. Certainly i am aware that the single cable of HDMI is designed with this in mind.

i was not aware that this was possible via DVI to HDMI. My tv is connected to the 990 via composite video for the 990s menu and a DVI to HDMI which i use to view programming and DVD. This also works with my DVD player which is connected to the other DVI input. Both the DVD and HD cable box use HDMI to DVI to feed signal to the 990.
i guess that i hadn't noticed this as i had never previously used the TV's speakers, they live in a box in my attic.
Is this an intended design feature, or are the 2 transmission methods more than just compatable at the video level?

No, To my knowledge,it's not possible to transmit
an audio signal via DVI. There has to be some other path that the signal is taking to reach the 990.

Others have reported that the 990's DVI switching can pass HDMI audio. It appears that the DVI-to-HDMI cable or adapter needs to be wired such that the dual-link DVI pins are tied to the HDMI audio (since HDMI video is equal to single-link DVI). How to identify such cables or adapters is problematic, but I think you are at least the third person to pull this off and comment on it in the saloon.

interesting.
it has to be via the DVI passing the signal.
there is no other path the audio signal can take to my tv, it is the last item in the chain & is only connected for video and has no signal return to the 990.

as for the cables that i've got hooked up for video, all are monster products. Sorry i don't know the model #s off hand. next time i break my system down, soon, i'll post the info.

they are not necessarily what i would have chosen but i got them all at 50% off when the local compusa closed up shop. Certainly a good move since i needed a 20ft run from the 990 to tv.

It sounds like your DVI-to-HDMI cables are using a dual-link DVI connector...