Unicode is not quite the right place for the line, I think. Cypro-Minoan is text (but not currently part of unicode). ASCII art is not text.
Linearity is part of it. Source code and emoji and simple math expressions can be seen as a linear stream of "characters" (defined loosely), even on a fundamental level. An architectural plan could be transliterated into a linear form, but that would be a transformation.
Of course then you need to explain why audio doesn't count as text. Discreteness? Ease of faithful reproduction?
Linearity is part of it. Source code and emoji and simple math expressions can be seen as a linear stream of "characters" (defined loosely), even on a fundamental level. An architectural plan could be transliterated into a linear form, but that would be a transformation.
Of course then you need to explain why audio doesn't count as text. Discreteness? Ease of faithful reproduction?