Via negativa:
Does not end with a P.
Not an ism.
Not a dream.
Not the absence of something.
Not the presence of nothing.
Not thought.
Not logic.
Not mathematical.
Not bad weather.
Not beauty.
Not a colour.
Not connected to music... (those look like significant dots)
Not a natural creation.
Not a behaviour.
Not an instinct.
Not spatially located.
Not entropy.
Not a law of nature.
Not photosynthesis.
Not embodied by people.
Not independent of Earth's existence.
Not hygge.
Not a view (whether vista or opinion).
Not observable with sight.
Not an extreme.
Not observable by electromagnetic radiation.
Not evil.
Not being foreign.
Not the last trump
Not the 2001 theme.
Not a song.
Not a sound that most of us are likely to have heard.
Not the music of the sphere.
Not on the Voyager discs.
Not traffic noise.
Not a sound made by a fluid.
Not the sound of an unheard falling tree.
Not an audio recording.
Not the sound of one hand clapping.
Not taiko.
Not the memory of a sound.
Not an echo.
Not only speech.
Not the Big Bang.
Not in a specific language. (But the first one was in French.)
Not to do with national anthems.
Via positiva:
Is a single word.
Could exist in the absence of life.
Artistic.
Observable.
Would exist if no-one were there to think of it.
Aesthetic.
A sound! (The first real progress.)
An artistic sound.
A sound made by a human.