Sound waves require a medium, such as air or water, in order to travel, and outer space does not contain a sufficient medium for those sound waves to be able to travel. When a sound is released from a source, the wave causes molecules in a medium to vibrate and collide with each other, resulting in a sound that ears can detect. However, since molecules in outer space are spaced too far apart to interact properly, vibrate and collide with each other, sound cannot be heard in space by the human ear.
It was formerly believed that space contained absolutely no medium, but recent studies have shown that outer space does in fact contain a small amount of molecules. However, these molecules are spaced too far apart in order to affect a sound wave and result in a detectable sound. NASA has developed ways to measure plasma waves, which can only be picked up by specialized equipment, with the deployment of Voyager I. While this shows that certain waves can travel through space, this does not mean that human ears are advanced enough to hear these "sounds," which are not technically sounds as they are commonly understood, but are waves of electrons in the ionized gas that the plasma detection instrument travels through. These plasma waves are also instigated by large events, like solar storms. In conclusion, sound waves cannot travel through space, since the molecules are spaced too far apart to properly interact, vibrate and collide with each other.