Last year, about 3/4 of the way through, I decided to do a little experiment on my middle school students. I knew already that some would be able to do what I wanted them to do and many would not, and that those results were probably more a function of prior experience plus ability to focus plus giving a toss than it was a result of my teaching.
I gave them a set of four Czech folksongs that only use tonic and dominant in major that basically everyone in the country knows, I erased the chords that were on the sheet, and asked them to figure out where the tonic and dominant chords should go. I had not been attempting to explicitly teach this skill, but was curious if anyone had figured it out. I allowed them to work together, which means that certainly some weren't really doing the listening and relying on others to do it for them. Some students absolutely just picked a random order. Some picked a progression that was non-standard (as in not the way it's usually played), but worked. Some knew exactly what to do on practically the first try.
Ovčaci
Skákal pes
Pec nám spadla
Pásla ovečky
Many of the ones who got it wrong, even when presented with "Here's the melody with this chord… Okay, here it is with the other chord…" couldn't really tell the difference; they couldn't hear that one sounded more dissonant than the other. Is it experience? Attention? Aptitude?
Recently, I read about the phenomenon of not being able to visualize, aphantasia, and I thought about my students not hearing what was obvious to me and some of the other students. Curiously, there are many people in visual fields (art, architecture — film, I'm sure) who have this condition. The simply have to work visual things out in front of themselves and not "in their mind's eye." Amusia is the inability to make sense of music; the composer Maurice Ravel suffered from this later in life (a real tragedy!). I don't think this is what my students who do not hear the relationship of melody and harmony have. What I'm curious about is whether there is also an inability to audiate. I would think that if that were true, learning to speak would be damned near impossible. But maybe not. Those are separate areas of the brain; one could imagine linguistic sounds but not musical ones. After all, there is plenty of documentation about people who, say, lose the inability to speak but are still able to sing even with lyrics.
This is one instance where I think Gordon's musical aptitude test might actually be worth it — why push someone to audiate if they simply cannot imagine sounds? Like the aphantasic animator, someone who could not imagine sounds might still be able to make sense of and make judgments about music as it's actually happening. It would not preclude them from participating in music making, as amusia does, but simply mean that testing things out would have to happen through actually making sounds.