Can you elaborate on this? I understand that there are many sources of entropy for random number generation, and it seems like this would be as good a way as any to store* a pool of some for future use.
(* Is it that the storage is the inherent problem?)
I meant it in the sense that it seems intuitively correct that you'd be able to get some random data out of a microphone somehow, but that intuition doesn't serve as a good adviser in cryptography.
I bet just looking at a recording from a computer microphone, it's possible to make deductions about it's structure. Let's say there are people in the room who talk with a certain pitch, the frequency of the AC current as it get's into the computer, things like that which all make the microphone output non-random.
So it might be that there is a way to generate true random numbers like that, but it's probably not as easy as just using the microphone as a source of entropy.