Holy cow. I haven't posted to this thread in forever.
The verb listen is a causative formed from the obsolete noun list, meaning "hearing" or "sense of hearing". So listen originally meant something like "to cause to hear". List comes from an extended form of the Proto-Indo-European root *klu/kleu, which became *hlus in Proto-Germanic. The h disappeared at the end of the Old English period, though I'm not sure where the t came from.
A passive participial form of *hlu gives us the adjective loud. The passive participial would have meant something like "been heard", so something that has been heard must be loud enough to not be drowned out by other noises.