<<He was at an angle, and his head was behind Obama, to whom he was speaking (Obama was between Biden and the mike). Also, it was picked up by mikes in the audience as well as those at the podium (youtube has some video taken from cell phones in the audience where it fairly clear).>>
Well there's obviously way too many variables and unknowns in Biden's situation to be of any useful comparison with the mikes in the crowd.
Depends on what you mean.
Most of the diffrences are things that would make catching Bidens Single Whisper harder to catch than the fifteen "shouted " "n" words.
The one variable that would make the Outdoor "N"s tougher to catch than the indoor "F" is the background noise being higher .
But this variable would necessacerily be equally against the hearing of intelligable words by the humans who claimed to have heard them.
I beleive you have already admitted that all of the other words spoken or shouted by the croud ,disapeared into the croud sounds and became impossible to interpret .
Too much ambient noise can mask a lot of words and make the noise to signal ratio too high to allow the understanding of words.
This is just as true for human beings alone as it is for human beings enhanced with electronic devices and for recording devices.
When assulted with "white noise" the human brain is able to fill in the gaps and attempt to make sense of the randomness. This Phenominon is so common that there is little reason to doubt it as a likely explanation for the hearing of shouts that no one could record.