Xbox360 to Dell All-in-one through HDMI cable, no sound

You can just get the standard cable, there is no real difference between the $3 Amazon.com cable and the $800 Best Buy cable (make sure you read the reviews on that one).

With digital-signals (unlike analog), lower-quality signals give you the same quality audio/video. HDMI cables only carry digital signals, so using ultra-high quality titanium-shielded solid-gold cables is no longer necessary to get good-quality audio/video.

You could use something that splits apart the audio and video data sent over the HDMI. something like an HDMI to RCA Component AV-Converter with a HDMI splitter perhaps, or if you have an unlimited budget then one of these should perfectly fit the bill.