Well, I would guess then that the music is nondiagetic. How can the actors hear the music when it hasn't been recorded yet? And how would the characters hear the music in the movie? Music is a tool to emphasize emotion within a film in order to highten the feelings the viewer gets when they watch a film. Look at psycho for example. The shower scene. Watch with the music and without. Its a hell of a lot more effective with the music than without.
What Hans Zimmer does in Gladiator however is use the Leit Motif. Which Wagner created. These are small snippets of melodies that identifies with character or an event. Look at Star Wars. Each character has almost their own theme. So when these themes are played, the audience in a sence recognises with the character. Anyway, back to main point.
The sound world is probably then Diagetic. However, the sounds the emphasize in the movie are always large in life than in the real world. You dont really hear major crashes of thumps or clanks when people fight. But it is increased to highten the intensity of the battle so that people get the sence they're almost there. Its also there because they can really record the sound good from location, so rather have clean sounds recorded in a studio instead, that can be mixed and matche din order to give the correct effect. If you want to see a good example, listen to a dogma film and then watch a hollywood action film. You'll see the difference.
In the end, its all about sucking the viewer in to accept the world they are presented as well highten the emotions and play with the audience's mind in order for them to feel certain things at the right moment. I hope this made some sence...
