ADAU1979 ФАПЧ и последовательный порт аудио часы
ADAU1979 PLL and serial audio port clocks3756" Общие вопросы
The ADAU1979 ADC MCLK and Serial port clocks needs to be synchronous else you will have sample slips and not recommended for audio as it will not sound good.
The serial port supports different sample rates but typically going down would not create an issue but going up would create images in the pass band. It is not clear if this is audio application or not. Again I would not recommend operating this way for audio application.
The PLL automute will mute the ADC output .i.e. it will produce nearly zero signal at the serial output. This is to prevent the audible artifacts in case of PL unlock.