File src/sounds/README.md from the latest check-in
[0-9a-f].wav files in this directory contain a human voice
speaking each of the 16 hexadecimal digits. If a captcha string
consists of just hexadecimal digits (as is the case for captchas
generated by the captcha.c module) then these WAV
files can be concatenated together to generate an audio reading of the
captcha, which enables visually impaired users to complete the
Each of the WAV files uses 8000 samples per second, 8 bits per sample and are 6000 samples in length.
The recordings are made by Philip Bennefall and are of his own voice. Mr. Bennefall is himself blind and uses this system implemented with these recordings to complete captchas for Fossil.