You definitely know about polyphonic music in films. It's when the background music does not complement the action on screen. The music, as well as the images, has an independent line of thought.
Imagine a scene wherein the city is under destruction. People are yelling and dying, and definitely in that order. You can put in homophonic music (homophony) that complements the images; sad music or anything that complements the doom on screen. Or you can put in happy music, that does not complement the images but 'does its own thing', that is, it is independent of the images on screen. Happy music and destruction on screen, that would be polyphonic music. Also vice versa. (Here's a violent example from Reservoir Dogs. One minute mark onwards.)
Polyphonic music was first written around the year 900 in London, as per recent discoveries. Let's step out of the
