Why the Jargon?

Students often wonder why we use such terms as "diegetic" and "nondiegetic" when filmmakers themselves rarely use that sort of vocabulary to talk about their work. David Bordwell offers a nice response to this complaint:

No doubt, we should be attentive to the ways in which filmmakers think and talk about their work. There’s a lot to be learned from shop talk and insider information–hence the enduring value of interviews, DVD commentaries, and the like. Yet no activity explains itself. Often practitioners do things intuitively, without making their background ideas explicit. We can often illuminate a filmmaker’s creative choices by spelling out the unspoken premises behind the work.

So the point of using (academic) terminology is (or ought to be) to illuminate something that cannot be—or is at least not easily—articulated in the vocabulary used by the industry. A productive answer to a student's complain about "diegetic," then, would want to explain how thinking about music's "source" can get in the way of, or obscure thinking about, its relationship to narrative.