Acoustic vibrations propagate as mechanical waves of pressure in a transmission medium such as gas, liquid or solid. The speed of sound in air at 20 ºC is about 343 m/s (1,235 km/h) and complexly depends on density and pressure/stiffness of the medium. Audio range falls between infrasonic (<20 Hz) and ultrasonic (>20 kHz) frequencies.
Sound is amplified and transformed into nerve signals by mechanically activated hair cells emitting glutamate neurotransmitter in a basilar membrane in the cochlea of the human inner ear. It happens in a spiral organ with 2.5 coils of tonotopically organized bone tissue resonating with different frequencies in its different locations.
Harmonics – the natural resonances
A note isn't just a wave, it's a mix of resonating modes of oscillations.