Wings of Desire (1987)

This film is well known for the complexity of its treatment of diegetic and non-diegetic sound, but that treatment is not at all hard to follow, because the distinctions involved are tied to familiar spatial (heaven/earth) and agential (angels/humans) dichotomies. Music, on the other hand, does not often participate significantly in this complex play (that is, take advantage of its own malleability in terms of the diegetic and non-diegetic). 

One interesting case to examine, though, is a modernist choral piece performed as we see the angels listening to people reading and thinking in a large public library (is the music simply non-diegetic, or is it, in turn, representing aurally the angels "thinking"?) (the scene begins at 00:16:20, the music at 00:16:53).