beeedy:
I found your elephant analogy offensive ...
No offense was intended. That's a common saying in my part of the United States; I suppose it's less common elsewhere. A better expression of my intended meaning would be, "The issues involved here are complex, and they would be difficult to explain, or to understand, all at once. We'll take it a step at a time."
It appears that this code was adapted from this link - FHTExample - Open Music Labs Wiki - and that it was transferred largely intact. The original code operated the ADC in a polled, free-running mode, without any interrupts involved. Adapting that code to an interrupt-driven application isn't trivial, and adapting it to run the ADC at a frequency that's not one of the prescaler's natural speeds is trickier yet.
Good luck with your project.