If a sound wave strikes your eardrums very often, it occurs with high frequency . A high frequency wave means that the sound will have a high pitch . We can tell from the sound of a train whistle whether it is coming toward us or going away. This is due to the Doppler Effect . In other words, to our ears they will occur more often, and therefore sound like a higher pitch.
Sound needs to travel from molecule to molecule to travel.
The further apart the molecules are, the slower the sound will travel. In fact, sound will not travel in a complete vacuum .