Linked from
The 9 pages that link to Audio signal, each with the reason it gives.
MicrophoneRelated: The microphone's output becomes an audio signal for processing or playback.
LoudspeakerNarrower topic: It is the input whose changing voltage drives the loudspeaker.
WaveformBroader topic: Audio waveforms encode the changing pressure patterns heard as sound.
SubwooferRelated: The subwoofer reproduces the low-frequency content carried by an audio signal.