I am actually having a similar problem capturing 360 footage through HDMI. When I turn on the xbox I get no sound. But if I fiddle around with the resolutions, it seems to spontaniously give me the sound back.
Say I was set at 720p and I changed it to 1080i, it would give me a message asking me if I can see it, then do the same at 50Hz. Then if I turn it back to 720p it will go through the same process but randomly start giving out the audio. I can’t even replicate it every time, and if I get the sound then restart my xbox, the sound is gone again. It’s very frustrating. I don’t know whether the xbox is faulty or the cable is fawlty or the intensity or the software.
Anyone have any idea why this would happen?