Humans cannot hear sounds in outer space, because in order for the human ear to detect sound, a sound wave must have a medium, like air or liquid, to push and travel through, eventually exerting that pressure on the eardrum and allowing the ear to hear the sound. There is not enough of a medium of molecules in space through which sound can properly travel, which is why the human ear cannot detect sound in outer space.
It is a common misconception that there is no medium at all in outer space. It is not that a medium is non-existent in outer space; it is simply so sparse — the molecules and particles making up the medium in outer space are so far apart and spread out — that a sound wave cannot properly cause a collision among the particles which is required to create a sound for the human ear to hear. NASA's Voyager I spacecraft was able to detect a wave of particles from the sun's solar winds. There are also plasma waves released throughout space that theoretically release sound. However, the human ear is unable to detect these particle waves and therefore would not hear them. These particle waves can only be detected by advanced and sophisticated machinery intended for that particular purpose.