I have been having this problem with my HD7970 since I got it.
I have the graphics card plugged into my Sony Theater system, and the 24in Asus LCD on my desk. The Sony system is connected to the computer over HDMI.
Every time I disconnect power to the Sony, or switch modes from HDMI computer, to anything else, the computer stops detecting the presence of the sony!!
This is remedied by re-installing the graphics driver pack. This wouldn't be a problem, but it's doing this 3 to 5 times a flipping week!!
At no time is the Sony ever being unplugged, only it's modes changed! Why does my cards HDMI sound output keep failing!?!
It's detecting the Sony's connection and the display works, but the soundcard fails to recognise the connected device!
Help! No other forum, or any other person can seem to give me a reasonable answer as to why it's doing this, or offer any solution.
My system details.
Intel i7-3930 (stock clock).
Asus HD7970 4GB. (stock clock) (drivers: 188.8.131.52)
AsRock Extreme4 Mobo (nothing unsual here) 16GB Gskill memory, all stock.
5 spin drives varying sizes, OCZ 160GB SSD. Nothing special.
Fairly straitforward build, i've done nothing to it that could affect this current problem. (the problem is almost certainly driver related!)