For reference, I've got an Xbox 360 Elite... I think (got it 2nd-hand. it's black with a 120gb HD so I'm pretty sure it's the elite) and am using an LG 32LD350 32" 1080p TV.
Anyway, my problem is in two bits:
1) I wasn't getting any audio when using my HDMI cable. This problem may still be unsolved, but now I have issue number 2.
2) In the process of trying to fix the audio, I disabled an option in the HDMI options menu on the xbox (something about automatically getting TV options over the HDTV cable) and now I'm not getting any image at all over HDMI.
So, in order... I need to get my HDMI image back (currently using AV which is doable but a bit blurry, so obviously I want to make use of HD), and then fix the sound.