Sound consists of audible vibrations, which travel from a source to a detector. These can be of a whole range of frequencies and a whole range of amplitudes. We hear these as sounds of different pitches and different loudnesses. Sometimes one sound will be made up of many of these frequencies at once, each with their own amplitude. All of these contribute to the amplitude of the sound, as well as its characteristic quality.
These vibrations travel at one quite constant speed, about 340 metre / second in air, whatever the frequency or amplitude. Sound travels through solids and liquids at higher speeds.