Audio signals may be synthesized directly, or may originate at a transducer such as a microphone, musical instrument pickup, phonograph cartridge, or tape head.
Loudspeakers or headphones convert an electrical audio signal back into sound.
Signal flow may be short and simple as in a home audio system or long and convoluted in a recording studio and larger sound reinforcement system as the signal may pass through many sections of a large mixing console, external audio equipment, and even different rooms.
A digital audio signal can be sent over optical fiber, coaxial and twisted pair cable.
A line code and potentially a communication protocol are applied to render a digital signal for a transmission medium.