Are you using a soundbar or any other AV device.
If the 360 is connected directly to the TV with no soundbar etc try a different HDMI connector not the HDMI-ARC connector.
It could be a HDMI handshaking issue, can you turn off the HDMI control on your TV to see if it makes any difference. Samsung call it Anynet+, and Sony, Bravia Link, etc.
If that fixes the problem you can get a 'CEC Less' adaptor from Amazon
