It is well known that if your working memory becomes more loaded you can easily miss visual ques. This is because precious space in your very limited working memory is taken up listening to radio/media rear parking sensors, passenger etc even subconsciously. When reversing you are also subconsciously calculating angles, closing rates, predicting where the vehicle will travel, sending motor instructions to arms legs and eyes etc etc. There is a lot going on that you may not consciously realise Hence the volume is reduced to allow the driver more chance in recognising danger or potential risks/errors