One of the best clues a cyclist has to his safety as measured by proximity of automobiles is engine noise. Cars are behind him, and even if he has a little rear-view mirror on his helmet the roar of internal combustion is an extremely helpful tip-off.
Riding with headphones is stupid and risky. Anything that robs a cyclist of any of his senses makes him more likely to be killed, and he should not rob himself. The things other people do to him are scary and dangerous enough.
Stop being boneheads, cyclists of the world. It lets people call you boneheads.