Why does sound need air in order to travel? I know that in space, because of vacuum that we can't hear voices or sound to be specific, why is that? Why does sound need a medium to go through?
 A: Sound is a pressure wave:

Each vibration compresses the molecules and that compression propagates onwards. In order for sound to exist, we need something to be compressed. In space, the particles floating about probably aren't close enough to compress and make anything audible (I haven't traveled to space to test this, and I don't think anyone who has gone to space has tried it either).
A: Sound, by definition, is vibration of medium. So, you can't imagine sound without a medium.
A sound source like Drum vibrates the air molecules making sound wave. When this wave reaches your ear, it vibrates your ear drum which is processed by your brain.
A: Because that's what sound is - waves, or oscillations, of a medium. 
Different things produce air (water) oscillations and humans, like many other animals, developed a device - called ear - that can discern these oscillations and our brains learned to make sense of it.
So no medium, as in vacuum, means no waves of a medium, and therefore no sound.
That's all there is to it :)
A: That's because sound is a mechanical wave. From Wikipedia:

A mechanical wave is a wave that propagates as an oscillation of
  matter, and therefore transfers energy through a medium.

Most notably:

A mechanical wave requires an initial energy input. Once this initial
  energy is added, the wave travels through the medium until all its
  energy is transferred. In contrast, electromagnetic waves require no
  medium, but can still travel through one.

Since a vacuum is defined as lacking a medium, then sound and correspondingly, mechanical waves, cannot propagate through a vacuum. This is why we can't hear voices or sounds in space.
