To hear a sound there must be vibrations, which travel from a source to a detector. These can be of a whole range of frequencies and a whole range of amplitudes. You hear these as sounds of different pitches and different loudnesses, if they are within the box of hearing. Sometimes one sound will be made up of many of these frequencies at once, each with their own amplitude. All of these contribute what we hear.
Vibrations of all amplitudes and frequencies travel at about 340 metre / second in air. Sound travels through solids and liquids at higher speeds.