Propagation of sound is the transmission of sound waves (or the travel of sound) through a medium. The speed of sound is usually around 340 meters per second (or 1 mile per five seconds) but it can vary based on the type of medium it passes through, the temperature, the air pressure, and the altitude.
Amplitude is the size of a sound wave, it affects the volume of a sound. Amplitude is measured in decibels. Amplitude of sound in the air is measured in dBSPL (or decibel Sound Pressure Level), with 0dBSPL being the quietest, while amplitude of sound in a computer is measured in dBFS (or decibel Full Scale), with 0dBFS being the loudest.
Amplitude vs Loudness
The terms amplitude and loudness are often used interchangeably, however, they are not the same thing. Amplitude is a set level that can be measured. Loudness is how the individual perceives volume. For example, one person may perceive the volume level of a television to be barely audible while another person perceives the same level to be too loud, this would be loudness. The volume range of the television would be the amplitude.
The frequency of a sound is how many times per second the sound wave pulses. The higher the frequency is the higher the amount of pulses per second will be, the lower the frequency is the lower the amount of pulses per second will be. Frequency is measured in hertz, with one Hz being equal to one pulse. The human hearing range is generally between 20Hz and 20kHz.
Timber is what enables us to differentiate between notes played on different instruments at the same pitch and amplitude. Also referred to as tone color or tone quality, timber is the collection of sound in multiple frequencies.
To summarize: Propagation is the transmission of sound waves through a medium. Amplitude is the size of a sound wave and affects the volume of a sound. Frequency is the amount of times per second a sound wave pulses and timbre is the collection of sound in multiple frequencies.