Sound Analysis

 Diegetic sound refers to sound that is in the world of the film or tv-show.

This includes: dialogue, sound the characters hear and music the characters can hear, ambient sounds, sound effect

Non-diegetic sound refers to sound that only the audience can hear and it's supposed to enhance the audiences experience 

This includes: voiceovers, underscores, music characters don't hear

Baby Driver:

Baby Driver uses the song Bellbottoms, it is used in a diegetic way as the main character has earphones, so only he can hear it. There are other diegetic sounds such as police sirens, car driving etc.