I've been reading a bit about the challenge-response anti-spam system.

Basically, when someone sends you an E-mail, and they are not on the
"allowed" list, an autoresponder sends an E-mail back requesting a
response, only using a picture of a number to verify that the original
sender is not a spam-generating marketing computer.  If the live person
reads the picture and sends back the number in it, the live person gets
added to your list of "people who are allowed to send me E-mail".

The problem is that when a blind person gets such a challenge E-mail,
that blind person cannot read the picture of the number.  The picture of
the number has also been altered to prevent a machine from using OCR
software to decipher the picture, which also prevents the blind person
from using OCR software to "read" the picture to get the number on it. 
This problem exists on free web-based E-mail systems as well.

Has any company doing challenge-response SPAM protection implemented some
form of audio-based challenge-response system?  I'm asking because I know
of a number of government agencies which are trying to limit their own
SPAM by using challenge-response systems, and it seems to me that any
system of challenge-response which does not have an audio alternative is
in violation of the A.D.A. or a similar law I don't know about.

The other point to be made is that any company which does implement an
audio alternative is automagically the only choice for a government
office and for any other company which can expect to have E-mail sent to
its employees from blind people.  And if such a company does not exist
yet...  :)

The audio file need not be terribly complex.  A spoken number followed by
a specific number of beeps and then another spoken number would be enough
to confuse the machine "listening" to it.  Or use a math problem using
very basic math operators: <spoken> "What is 200 plus 4 plus 30?"  If the
blind person returns 243, they get added to the "allowed" list.  A
machine "listening" for numbers would probably return 200430 and be
rejected, if the spammer wanted to go to all that trouble.

Or use the time-honored method in science fiction for letting your alien
captors (who presumably don't speak English) know that you are
intelligent: tapping out numbers on a wall (Picard did it once with the
prime numbers).  Use an audio file in which a series of clicks followed
by a beep denotes each number.  People count the clicks and stop counting
for each number at the beep, and send back the number they heard.  This
gets around some of the language-barriers.

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