I was curious myself, so did a search as I have never used that feature (yet).
In general:
A (playback) audio-file can linked to 1-4 Multipads. So indeed a buttonlight should be burning if he used a Multipad with audio-link. The audio must be a WAV format (samplerate 14.1, bitresolution 16). It can be stored on USB or USER Memory.
I can't find how large the Wav-file can be??
In the Vid. of Yamaha Global (those with Harris) there is a demo-vid. As well in the Vid. with Peter Baartmans in Vietnam, he showed/demoed the audio-link.