One of the basic questions for nonduality in general and for Advaita Vedanta in particular is: “How can a changeless, partless Brahman undergo change?” The basic answer would take us deep into an account of unreal causation.
For now, let’s simply turn to Ananda Wood’s words from his Ways to Truth: A View of Hindu Tradition (2008):
The illumination of consciousness [i.e., the self-luminous nature of Consciousness] is not a changing act, which is put on at one time and taken off at another. No action needs to be put on, for consciousness to shine. It does not shine by any changing act, but just by being what it always is. It shining is thus changeless, and involves no movement in itself. Appearances are lit by its mere presence, as it stays unmoved within itself. (p. 153, my emphases)
The above concerns the nature of light, or luminosity. What about sound?
So also the vibration [the sound quality] of consciousness. As it bursts out into appearance or draws back in, it seems to change; but the change is only in appearance. Outwardly, a change appears; but in itself, consciousness remains unmoved and unaffected, just as it always is. As differing appearances keep getting manifested forth and drawn back in, each manifests the unchanged nature of consciousness. As the cycle keeps repeating, it is just a repetition of that unchanged nature, over and over again. (Ibid)
1. Brahman is self-luminous and self-vibrating. Mere appearances of changes can be explained by appealing to a vantage point of a finite mind. For a finite mind, Brahman appears to change, appears to throw forth light, appears to vibrate in space and time. But in the absence of this vantage point, there is only Brahman Itself as Itself.
2. The above paragraph lays out one light of thought. Another is that, of course, Brahman’s self-luminosity would shine, Its self-vibrating nature vibrate. But nothing changeable is required of self-luminosity to appear to throw off light just as the sun requires no further luminosity in order to shine. That’s what the sun does: self-shining, it shines forth. Or–in the case of vibration–imagine that a drum’s single membrane had a natural tendency to vibrate. Then nothing further would be required of the membrane in order for it to vibrate: self-vibrating, it would simply, “generously” vibrate forth. In neither case would the self-luminosity or self-vibratoriness need to undergo change. It would remain unchanged, “unmoved in itself” and by itself.