Can't answer on when virgin or liberty will fix this HDMI-ARC problem, that's if they are actively looking into the problem, they say there are tickets open on the problem and they have the same problem on the ziggo Netherlands website, (all liberty owned), the only suggestion I can make is connect the 360 directly if possible to the soundbar.
You should still be able to use the HDMI-ARC connection between the TV and soundbar for audio between the TV apps and soundbar, if the 360 is plugged straight into your soundbar it 'should' be OK, if it isn't try using a non arc HDMI connector on the TV, and then we go from there.