Analog-to-digital converter (ADC)
The circuit that measures a voltage at regular intervals and writes the answer down as an integer — the doorway between the microphone and everything digital.
Two choices shape every conversion: how often the signal is measured (the sample rate) and how finely each reading is graded (the bit depth). Anything above the Nyquist frequency has to be filtered out before measuring, or it folds back down as phantom tones that no later filter can tell apart from the music.
Designs vary with the job. Flash converters compare against hundreds of references at once and live in oscilloscopes; SAR converters home in on the answer one bit at a time and sit inside microcontrollers. Almost every audio converter since the 1990s uses delta-sigma modulation instead: a one-bit measurement at several megahertz, heavy oversampling, and a digital filter that decimates down to 48 kHz. That moved the hard analog filter into the digital half of the chip, where it is cheap and precise.