Which movie(s) do you think has the best soundtrack?
I think American Psycho has a good soundtrack and I listen to it occasionaly.
The adjective for this type of music is “diegetic”. That’s sound which is occurring and audible in-universe, not just to the audience.
My favourite example of this is in Grosse Point Blank, when the GNR cover of Live and Let Die is playing non-diagetically, right until the moment Martin walks into the store, and suddenly a cheesy muzak version of it is playing over the shop radio. It’s beautifully done :-)