[pdkl95]

> what emotions are evoked by certain keys

Yes, choice of key is one of the tropes that is useful when composing a song's "plot".

> Music is not something that is reducible to mere quanta and waves and frequency.

That's my point; interesting aspects of a song are not derived from specific sounds (and their frequency/etc). Those are the atoms that can be used to create the larger plot.

While it is possible to reduce music to the frequency and timing of its atomic structure, it's similar to analyzing the phonetics of speech or the glyphs of text in isolation. A low level perspective may be useful, but misses the larger structure we call a "song" or "essay".