Okay, looks like there is CS8416 SPDIF/TOSLINK receiver chip on the dac board, probably operating on Hardware Mode. Looks like the only signal that might help you know when to mute would be the _AUDIO signal on pin-15. It appears to change state when a valid audio signal is being received. Would have to check it with a scope to see if it toggles just before a pop occurs. If so, it might be useful for controlling an a mute circuit or relay at the analog output of the dac.
Other than that, in Software Mode one can look at the PLL status to see if its locked. If not, then you might want to mute. However, accessing that would require an MCU, maybe such as an Arduino, to see if that status bit could be used.
Other than that, in Software Mode one can look at the PLL status to see if its locked. If not, then you might want to mute. However, accessing that would require an MCU, maybe such as an Arduino, to see if that status bit could be used.