very strange, i had this problem, my audio on my dell doesn't play so I had to purchase a VGA HD cable for my 360, this cable has RCA outputs instead of optical output, and also a jack adaptor to connect to a standard jack input on your pc, personally i like using that and you can set your own resolution compared to the HDMI cable. You may want to look into this cable >
Xbox 360 VGA HD AV Cable Review (Xbox 360) < plus I am aware the resolutions are shown on this website, there has been an update to allow all standard pc resolutions including 1080p