There is already a project whose name sadly escapes me that does this, using information theory to derive lower bounds on the number of bits of entropy in data read from a line-in jack.

EDIT: I'm thinking of Turbid: http://www.av8n.com/turbid/paper/turbid.htm

EDIT2: There's also a wealth of information in this thread from the metzdowd cryptography mailing list: http://www.metzdowd.com/pipermail/cryptography/2013-Septembe...