I've tried it; you don't get anywhere near that much entropy. Quite a lot of it is strongly
correlated, at least with cheap sound cards, even at the least significant bits.
I wish this wasn't true, but it is :/ or at least it was in 2003, when I last looked at it (I
still don't have any sound cards newer than that: I'm not made of money and the newer cards
don't offer anything I can use. I don't have enough speakers or a suitably-shaped room for