Diegetic and non-diegetic sounds

Diegetic sound is sound which is visible on screen, some examples of diegetic sounds are:

• voices of characters 
• sounds made by objects in the story 
• music represented as coming from instruments in the story space

Non-diegetic sounds is sound whose source is neither visible on the screen nor has been implied to be present in the action, some examples of non-diegetic sounds are:

• narrator’s commentary
• sound effects which is added for the dramatic effect
• background mood music