In this case, as Anup has explained below, there is no timing link between the source material and the DAC. The DAC does not have any other clock domains to worry about here, so it simply converts the audio samples from it’s buffer at it’s own rate. If there is a Master Clock in the system, this means that the DAC then has a cleaner external reference clock signal to lock to, ensuring the timing of when these samples are converted is good and reliable.
Using a Master Clock in this case does not solve any problems, but it does increase audio quality (see the Intrinsic Jitter post for details on how).