The speed of sound deals with molecules hitting each other one after another in space. So, in a vacuum, you won’t hear anything, if it is truly a vacuum. Also the closer the molecules are, the easier it is for sound to travel so sound will travel faster if it is colder or if the pressure is higher. When it is colder, the average distance between molecules is smaller and if the pressure is higher, the molecules are closer together – thus sound propagates faster.
On another interesting note, Lower frequencies travel farther. Elephants can talk to each other miles away using very low frequencies.