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If Quicksilver runs at speeds faster than the speed of sound then how come the song he's listening to on headphones is playing at a normal rate?
He can hear because the headphones will be moving at the same speed as him, thus will be projecting sound at a normal rate relative to him. As the headphones could create a sealed container of air between the speaker and his ear, the air inside would be relative motionless to both the source of sound (the speaker) and the recipient (his ear). Consider this article on Felix Baumgartner breaking the sound barrier.