i use an hdmi to dvi to hook up my cable box to my 970 and an optical cable for sound. was working fine until recently.

been getting a hasty humm when i turn my cable box on. nothing but humm. i played with the cable box settings to output audio to HDMI setting from the Dolby Digital setting. I do not know why but it worked. No more nasty humm. but now i don't get dolby digital output anymore. what is going on? is the cable box messed up?