Seems like they would've recored both in the field, no? If they were recording sap freezing in the field, presumably the mics would pick up on other parts of the tree undergoing stresses and making audible sounds.

For that to have not been the case, either they would've had to freeze sap in the lab, or they would've had to go way out of their way to isolate recordings of just the sap in the field without the rest of the tree (is that even possible with normal recording tech?)