When someone is seated at the piano, we are generally not debating whether person A is hearing a piano and person B is hearing an organ or a flute. There is little debate and imagination as to a piano being a real thing, and its sound being a real thing.
They do imagine it. Our senses are imaginary at a very basic level. That’s why a machine doesn’t replicate what we “hear.
And hence the measurements tell us how closely a playback of say a piano sounds like, say… a real piano.
We do not care whether person B hears a piano sounding like flute, we only care that it is replicated correctly.
To talk about brains and personal experiences is delving into psychology and neuroscience, whereas the playback system is electromechanical. We have to draw the line somewhere.
To talk about subjective differences in hearing, is burying one’s head in the sand as to the reality of what is happening.
That thinking belongs in a course on philosophy and not in engineering or physics, and hence it belongs in a coffee house and not in an audio shop or engineering laboratory.