Okay, I will give a serious response to this. We are not dogs, our sense of hearing is not the same, we cannot discern location well from sound, as dogs, and most animals can. While I believe you should be able to hear sounds around you, such as horns and emergency vehicles, I also do not believe you can use sound to pinpoint a car coming up behind you too close to the edge of the road to be safe. Sure, you may hear a car coming up behind you, but so what? where I ride, that occurs almost every second, so what do you do, pull over and stop? The entire argument is ridiculous.
I also do not believe one should blast music loudly in earphones, for many reasons, one of which is to not shot yourself off from noises you do need to hear, like the above mentioned emergency vehicles, sirens, and horns. I have used earphones to listen to audiobooks on tour, but I could hear other noises easily.
If you want to know what is behind you, and whether or not it is a danger, use a mirror. Until you learn echo-location, or develop a dog's sense of hearing, relying on your ears is not an option.
To believe you can know a car is coming up too closely based on hearing is ridiculous. Ride in a town with heavy traffic, all you have a cars, and you will hear all of them. Which one is a danger? Based on hearing, you will not be able to know. You need to see. The only real issue is that many who wear earphones, are simply inattentive to the world around them, that is another issue entirely.