Arnold Schoenberg observed the deep connection between music, perception, and cognition; anticipated composers applying timbral control as is now commonly done with synthesizers; and apparently did not anticipate non-subtlety of the effect in EDM: "I think the tone becomes perceptible by virtue of tone color [timbre], of which one dimension is pitch. ... Pitch is nothing else but tone color measured in one direction.…
Maybe a reason for calling music an illusion is to try to point out that it's all in our heads. Sure, the physical instruments are over there, but they are just some atoms and molecules rattling about creating vibrations that, like everything ultimately, share the meaninglessness of the physical universe of just more material stuff (ignoring the quantum and the unknown). In which case though life seems also to look more like an illusion. Ah but the meaningless vibrations enter our ears and are processed and become music, so like color there is none but in our heads, thus illusion? But these notions also fall apart. The redness of apples isn't all in our heads; it requires a certain relationship to light frequencies that we see as redness. It's not an illusion it's just what red looks like through a living human body. If not through our senses and brain, and then mind, where should we experience it?
Music is only ever meaningless if it is unheard, as apples can only not be red when they are unseen (or are Granny Smiths).
So music is only meaningless if there is nobody -- no body -- to listen. And maybe that's where all of this leads. Your AGI needs to be embodied and alive.