I assume you are connected from the 360 to TV without a soundbar, what you could try is when pairing the remote doing the volume key procedure is the first time the volume changes say no, you might find another setting that will also work, there will probably be more than one.
Also if you are connected to HDMI 1 this is probably a HDMI-ARC connector you could try using another HDMI port to see if t hat makes any difference first.