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How do you add a sung part of a movie script? When the song is unknown and the lyrics are unknown, how do we mention there's a song playing in a movie script?
This image gives you a basic idea of what to do, but doesn't tell how to handle certain specific cases.
In general, you don't. In a movie script, unless it is relevant to the plot, you do not mention the songs that are played/music that's running. This is a job for the film score and director, not the script.
If, however, the song is relevant to the plot, then you can simply use capitalization, just like with all sound effects:
Zach turns to the BOUNCER. ANNA'S SONG is playing.
