Found the classically trained musician.
Any and all formal, mathematical, informal theory about music can be called music theory. Music theory is about modeling music. Period. It does not matter if it's about harmony, rhythm, pitch, form, timbre, dynamics, or some other aspect of music. Whether it helps with understanding music, composing music, improvising music etc is a separate topic. Music theory neither has to be about practical music making skills, nor it has to be about music of a particular artistic tradition. It just needs to present some model that can be a helpful tool in some musical context. Maybe what you call "audio engineering" is a specialized skill for some musical traditions, but for musical traditions where the expressive content primarily comes from timbre and synthesizers are common instruments, it will be an essential music making skill.
I think there is just a certain a kind of ambiguity with the word "Theory". Miller is really focused on the theory of sound synthesis and does not really deal with composition or aesthetic theory. People who are more interested in the latter might enjoy "Composing Electronic Music: A New Aesthetic " by Curtis Roads (https://global.oup.com/us/companion.websites/9780195373240/b...).
I think in order to have a book about music theory, there should be some explanation as to how to make music for some particular expressive purpose and not just the technical details of how to make sounds. A guide to constructing a piano is not music theory. I don't care if it's classical music or techno or gamelan, or if the theory is formal or traditional, if there's not some discussion of how and why to express musical ideas, it's a technical manual and has very little to do with music.
The traditional concept of notes (and accessory ones like scales and traditional notation) marks the boundary between "traditional" music theory, that treats notes as the final result (when you have notes written down, it's only a matter of concretely playing them with a given instrument) and theory of electronic synthesis, which treats notes as an input, both optional and taken for granted, and audio signals as the product.
Huh? There's a huge amount of theory and writing about electronic music that isn't just technical. See Mark Fell's PhD thesis, for example.
Your comment seems to suggest the other person is ignorant but really it just shows your ignorance of theory and writing about experimental and electronic music. Not all music theory is western classical.
I mean, how do you even consider Stockhausen and Xenakis from your perspective?
> musical traditions where the expressive content primarily comes from timbre and synthesizers are common instruments
What traditions are you alluding to?
Nevertheless there is a huge difference between composing or improvising a musical piece, and programming a filter or oscillator.
Btw. "audio engineering" is what audio engineers are doing (see e.g. https://aes2.org/), and yet another completely different profession.