I'm beginning to not care for these Motorola boxes. They could at least label the outputs for pity's sake. Coaxial digital uses an RCA connector just like analog audio, composite video, and componetn video. The orange color is typically indicative of coaxial digital audio - yellow is composite video, red and white are analog audio. I think you're right.

Since the digital signal really is missing in action, here's a really quick test - unplug the HDMI cable and see what happens. I'd be curious to know if the audio starts up.