The diegetic music can be differentiated from the non-diegetic through different features:

• genre
• interpretative style
• auricularization/filtering
• diegetic presence of the music source

Transition from diegetic to non-diegetic with a change of musical genre in  The Journeyman

Two appearances of solo cello in Grimm 2×06: preexisting diegetic (Bach suite nº1) played with clear instrumental attacks and gestures, versus extradiegetic (uneventful performance)

http://politube.upv.es/play.php?vid=47262 Erik Satie, an extradiegetic composer

http://politube.upv.es/play.php?vid=47168 Chopin, a diegetic composer