Digital-to-analog converter (DAC)
The circuit that turns a stream of sample values back into a voltage a speaker cone can follow.
The textbook design is the R-2R ladder: a resistor network in which each bit carries half the weight of the one before. Philips' TDA1541 from 1985 was one, its 16 bits demanding resistors matched to within a hundred-thousandth — which is exactly why they were expensive. The output is a staircase, and a reconstruction filter sands off the steps so the original waveform reappears.
Modern audio DACs are nearly all delta-sigma designs, running few levels at a very high rate with noise shaping pushing quantization noise above the audible band. A chip costing pennies now measures better than 1990 studio gear, which hasn't stopped the hi-fi market from selling discrete R-2R ladders again, this time as luxury goods. In practice the clock's jitter and the analog output stage usually matter more than the converter itself.