Sound is an essential element of any video game. It can adhere both functional or aesthetic purposes. 

Source of the sound

Sound can appear in the game from different sources: diegetic and non-diegetic.

Diegetic

• World - ambient sounds
• Character dialog
• Character, NPC and enemy actions

Non-diegetic

Non-diegetic sound has a source external to the context of the story, and it is not heard by the characters.

For example, UI notifications or background music.

Purpose

Informative

Give info about object/subject like:

• Character state
• Success or failure of the action
• NPC and enemy AI tells comment on what they are doing and at what state they are - it is often called Barks.

Aesthetic

Make player feel certain emptions, get you in the mood, understand the theme

• Immersion into environment