Ask a room to match tones to brightness and the votes pile up: high goes bright, low goes dark and large. These shared intuitions are cross-modal correspondences, and they appear across cultures, in infants, and even in some other species.
They differ from synesthesia in strength and character: correspondences are biases, felt as fitting rather than perceived as color. But they map suspiciously well onto common synesthetic patterns, which is why researchers treat them as the population-level shadow of the same machinery.
Live visuals lean on them constantly. Mapping loudness to size and treble to brightness reads as natural to an audience because the audience arrived pre-wired to agree.