Did you test the audio output on your STB with the RCA? Or did you test it with HDMI?
This might help as a guide. There could be a setting in your cable box, such as optimizing the audio.
My SB 500 audio is no work either...I have tried new component cables. Did a cold reboot at least 6 times (each time it did a new software update). Nothing seems to make the audio come back. My theory is that one of the recent software updates SB did has a bug in it that effects the Audio....I'm sure SB will deny it! I now have a funny looking, $200 plastic paperweight. LAST SB PRODUCT I WILL BUY.