How Would I go About Making Custom Code to Check if a Jukebox is Playing Music?

       I made a post in the past about "how to sense if a jukebox is playing music" and learned that I would need to do some custom coding. The problem is, I don't know where to begin! I am trying to make it where on my mob's update tick, if it senses a jukebox playing music, it plays a custom animation. How would I do this with custom code? Thanks for your time.