Diegetic vs Non-Diagetic:
Sounds heard by the characters and are found within the environment
Sounds that are added in post, such as music scores
Voice Lines: Characters voice lines are diegetic, as some of their lines are call outs to their team mates.
Gun Shots/Ability Sounds/Character Sound Effects: The actual sound of weapons firing and the abilities being used, as well as foot steps, are sounds that are within the actual game and are heard by other players.
Environmental Sounds: Sounds such as weather, car alarms and things breaking are within the game
Announcer: The announcer itself blends between diegetic and non-diegetic. At points it appears that they voice lines are directed at the actual characters but usually it is there for the player. One good example is PoTG (Play of The Game) which happens as the post-match and would not be something that the actual characters would hear.
Capture Sounds: The sounds of capture points being taken would most likely be non-diegetic and would only be heard by the player, not the actual characters.
Menu/HUD Sounds: The sounds of options being toggled and heroes being selected is added in, only the player hears these sounds.
Music Scores: Music, that isn’t Lucio’s (he is a music-oriented character and creates beats in game), is non-diegetic and the characters do not hear this, only the players. The music changes throughout the matches and are influenced by the timer for the match.
Environmental Sounds: Sounds such as birds, weather, rustling of leaves and water would be heard by the character within the game.
Most Sound Effects: The sound of weapons clashing and arrows being fired, as well as the roars of dragons, are diegetic and are heard by the character.
Speech In-Game: The communication between the character and the NPC’s are diegetic and play an integral part in the gameplay.
Crafting Sounds: The sounds of crafting merge between diegetic and non-diegetic in the sense that the sounds of crafting are heard from a menu. These might not actually be head by the character but rather only by the player with the sounds being pre-recorded and played when an option is clicked.
Bards/Bard Songs: The music played by bards is diegetic. It is heard by the character and NPC’s in the game. This merges with the speech topic.
Menu/HUD: The sounds of the player toggling and selecting options are non-diegetic sounds.
Some sound effects: Sounds effects such as the noises heard when receiving a negative or positive buff would most likely not be heard by the character.
Music: The musical scores as well as the music played during battle and certain sequences are not heard by the character, but rather by the player. (This excludes the music sung and played by bards.)
Loading screens: The sounds and music from loading screens and the main menu are non-diegetic and are heard only by the player.
Summary and Differences:
Both Overwatch and Skyrim share similar points in regard to Diegetic and Non-Diegetic sounds. In both, music is usually an over-layed score with characters in-game communicating and/or singing with diegetic sounds.
In both games, diegetic environmental sounds are present with non-diegetic menu/HUD sounds.