> But sound quality in general is absolutely objectively measurable.

Sound quality is not the same as music quality.

To be more specific, Sound Reproduction Fidelity is not the same as Pleasant Music

To be even more specific, Signal Reproduction is not the same as "Pleasant Sounds*

The goal of music is not always high fidelity of reproduction; if it were, over-driven valve amps would never have been a thing.

The only thing objective in this context is signal reproduction, which is not the highest concern for music production.

You can certainly measure it, but the catch is that there is not always a single "correct" value. So just because you can measure what the speakers are outputting and then adjust it, it doesn't mean there is one correct output value.

A good example of this is a target curve, often used in room calibration. Dirac has a good explanation: https://www.dirac.com/resources/target-curve

(highly recommend Dirac room correction, by the way)

There's arguably a subjective quality to sound enjoyment, though. The fidelity of reproduction can be measured, but I'd argue there's personal preference in the types of artifacts generated by inaccuracies in reproduction.

you can absolutely quantify certain metrics, and you can even generalize what "good" is by surveying listener preference but that isn't the same thing as any one individual's subjective preference.