A key concept in the field of sound, sound reproduction, and sound reinforcement is the that of the transducer.  A transducer is a device which converts energy of some form into energy of another form. 
Musical instruments are good examples of transducers. The guitar, for example, is a type of transducer where the energy of the plucked and vibrating string is amplified by the wood of the soundboard which is then converted into vibrations in the air which we perceive as sound. Microphones are another example of transducers. In this case energy in the form of sound waves in the air are converted into motions of the diaphram of the microphone which are in turn converted into fluctuations of electrical current which can be carried by electrical wiring to amplifiers or recording and processing equipment.
Loudspeakers  are examples of the reverse process of transduction where energy in the form of variations of electrical current carried by wires are converted into corresponding variations in the magnetic field of the speaker magnet which results in turn in movements of the speaker diaphram which cause fluctuations in air pressure or sound.