In the review article “Hubbub”, Nicholas Spice answers:
[T]he most poignant encounters with music are inadvertent and unplanned. Church bells heard across the fields on a Sunday evening, the forlorn plinking and plonking two streets away of an ice-cream van on solitary summer afternoons, someone practising the saxophone in a neighbouring house: such half-heard music sets up momentary perspectives on our situation, touches us with sadness or strikes us with interesting incongruities. It is the literary imagination which is stimulated by music heard by chance, the imagination that enjoys the possibilities suggested by the collision of disparate realities, the imagination that feeds on the ironies which a split attention (not a distracted attention) perceives. (pp. 135-136)
For me, everytime I hear some fragments of the song “Streets of London” being played by a street musician on a London street or on the Tube, I am inexplicably sad. The music reminds me of a younger self listening to the song on the radio late in the evening in Hong Kong, many times. I had not dreamt that one day I would be wandering through some London streets, ‘Near where the charter’d Thames does flow‘.