Clock

A digital audio signal consist of two components, the value of the signal and the time (sample rate).
When converting from digital to analogue, the bits must be converted to a voltage of the appropriate magnitude. This is done by a DAC. The DAC is driven by a clock. The clock takes care of the timing.
According to the Redbook audio standard the clocks frequency  should be within +/-100ppm (parts per million). A deviation of 100 ppm means that a 440Hz tone deviates +/- 0.044Hz.
Far more import for audio is the cycle to cycle stability, each sample should be delivered with exactly the same interval. The deviation in this interval is the clock jitter.
Best results are obtained by using a crystal. Its frequency can be controlled by a voltage so a VCXO (Voltage Controlled Crystal Oscillator) is what you need.
Output jitter can be below < 3ps rms but the actual value depends on the noise generated by the power supply.
Price for this marvel of precision:  € 30,- .