Most or all songs featured in a work's soundtrack are also played from inside the work's universe.
Lots of films and TV shows have soundtracks: that is, audio matched with the video to contribute to a mood or feel. Some soundtracks, however, actually exist within the universe of a work.
These songs are known to the characters, and may exist on some personal music device of one of the main characters. This may be an object of incredible personal value, and the songs on it reflect the personality and character of whoever owns it — the character will choose songs to accompany current circumstances of the plot, and a particularly memorable song choice nearly always contributes to the awesomeness of a Signature Scene.
In other circumstances, it may be a case of a separate device — like a television or radio — or the setting of a club that allows the entire soundtrack to be diegetic. This may cause Suspiciously Apropos Music if the lyrics directly connect to important aspects of the story. If the music is initially thought to be non-diegetic and purely audience-level soundtrack, then is revealed to be heard by the characters, too, it is Left the Background Music On.
This can be accompanied by Diegetic Switch, as required to fit the needs of the story. It should be noted, however, that this trope refers specifically to the use of an item in the story to deliver the music — it does not need to be a diegetic switch nor are all examples of a diegetic switch examples of this trope.
A subtrope of Source Music and Reality Has No Soundtrack. For works with theme tunes, it will almost certainly overlap with Theme Tune Cameo. It may also be related to Celebrity Paradox insofar as real music, and thus the performers, exist in a fictional work — it can overlap with this if some of the performers on the soundtrack also play fictional characters in the work.
Because the songs are obviously real and the characters treat them as real, this trope can only be used in works where This Is Reality. Very little Leaning on the Fourth Wall is permissible. For the levels of diegesis within Musicals, see the Musical World Hypotheses.
Note: works with real-song soundtracks, and works in which real songs are simply played by/for the characters, are Not an Example. To be an example, the work must both have a majority of its soundtrack be pieces of music which are diegetic (playing from within the work's universe) and have these pieces of music function as a soundtrack (play in the background and underscore the scene).
Works where only one (or very few) of the soundtrack pieces are played in-universe are most likely to be an invoked and discussed version of Awesome Music (e.g. a character switching on cool music for a certain fight, only once) but may also be an example of Leitmotif or, because of the lack of being established as diegetic, examples of Left the Background Music On.
- Guardians of the Galaxy is the Trope Codifier for the modern day. Peter Quill, the protagonist, owns a vintage Sony Walkman that he uses to listen to a cassette tape filled with 1970s pop music that his late mother loved: "Awesome Mix Vol. 1." Do not mess with that tape. Highlights on the soundtrack include "Hooked on a Feeling" by Blue Swede and "Come and Get Your Love" by Redbone.
- Guardians of the Galaxy Vol. 2 features another diegetic soundtrack and explores additional ways that characters listen to the music, such as homemade* portable loudspeakers and a gigantic spaceship's intercom system in addition to the headphones and tape deck used in the first film. And after Peter's Walkman is destroyed in the film's climax, he is given a Zune with new music on it. Highlights include "Mr. Blue Sky" by Electric Light Orchestra and "The Chain" by Fleetwood Mac.
- High Fidelity: Protagonist Rob Gordon is the manager of a record store, and most of the songs in the film soundtrack are played In-Universe by him and his employees. This includes a cover of Marvin Gaye's "Let's Get It On" performed by Jack Black, whose character Barry sings it in the film.
- The Martian: Trapped alone on Mars, the only music Mark Watney can find to listen to is a collection of disco music files on Commander Lewis's laptop computer. He uploads the music to his rover's onboard computer and listens to it throughout the film even as he complains about it. Highlights include "Hot Stuff" by Donna Summer and "I Will Survive" by Gloria Gaynor.
- Baby Driver: Baby listens to songs on an iPod almost constantly to help combat the tinnitus that he otherwise experiences. Of course, that's some incredible Understatement, as the film blends music into its action in such an incredibly epic fashion that it's difficult to describe with just words. Highlights include "Bellbottoms" by the Jon Spencer Blues Explosion and "Brighton Rock" by Queen.
- In The Tuxedo, Jimmy wears a high-tech tuxedo capable of granting the user abilities. Whenever Jimmy selects one of the tuxedo's modes, different music will accompany the selected mode and then continue playing as the background music.
- Reservoir Dogs' soundtrack is selections played by a radio station's "Super Sounds of the Seventies Weekend" the characters listen to.
- Black Mirror: San Junipero features a soundtrack of awesome '80s (plus some '90s and '00s hits) that are played at a nightclub in which most of the episode is set.
- Most games in the Grand Theft Auto series have an In-Universe Soundtrack in the form of in-game radio stations that can be played whenever driving a vehicle, featuring a variety of licensed music.
- Metal Gear:
- In Metal Gear Solid 4, Solid Snake can listen to music through an in-game iPod, and can collect more songs at various points of the game. Most of it is the original soundtrack from various entries of the Metal Gear series.
- In Metal Gear Solid V, Big Boss can also play music tapes on a Walkman device through the iDroid menu, and can find more tapes scattered throughout different areas. About half of the music comes from the original soundtrack, while the rest are various real-life licensed songs from The '80s.
- All of the music in Splatoon and Splatoon 2 are played by in-universe bands, with your character hearing said music through speakers (like in the various shops), radio (stated to be the case in Hero Mode), their own music player (implied to be the case during most multiplayer matches), or live concerts (mainly during Splatfests).
- Zootopia: Most of the actual soundtrack songs are coming from the world of the film, like Judy playing the Gazelle song to give a city montage, the short car chase radio, and the final scene being a concert.