Underproduction is simple. You'll notice it when you find yourself lurking on the 'Net to find something that would suit your desires, and find none (or spend a long time until you find something). When there are too many such cases (so it's not just picky guys lurking for rare things) — it would mean that there's an underproduction.
For example, at some time you feel a desire to watch some hard sci-fi present-to-past accidental time travel causing an alternate war history movie¹, but don't know any, except for those you've already watched. And you fire off a new tab and start lurking around.
Not exactly kids on the steets begging for MP3s, but someone spending considerate amount on time begging^W searching for something they want, but don't know.
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1) Hopefully, that — while still being somehow generic — would be a quite narrow definition where not too many movies would fit.
At the extreme end: imagine the world without any commercial:
* Software
* Pharmaceuticals
* Books
* Music
* Movies
Now, I'm not saying that we'd arrive at that extreme, but you get the idea, no? That's "underproduction" in my mind - it'd be a poorer world with drastically smaller quantities of those things.
