1. A computer implemented method to authorize access to a computer function, comprising: receiving a request to access a computer function from a user interface; generating a Completely Automated Public Turing Test to Tell Computers and Humans Apart (“CAPTCHA”), wherein the CAPTCHA comprises: randomly selecting a sound category to be used as a validation sound, selecting a plurality of non-validation sounds, assembling a series of sounds that includes i) at least two validation sounds from the category of validation sounds, and ii) the plurality of non-validation sounds; establishing a time duration for the CAPTCHA, determining expected time intervals between the first validation sound and subsequent validation sounds; presenting said CAPTCHA through the user interface; recording a first input in response to a first validation sound and starting a timer for evaluating inputs; recording subsequent inputs during the test until expiration of the time duration of the CAPTCHA, determining time intervals between the first input and the subsequent inputs; and allowing access to the computer function if the time intervals between the first input and the subsequent inputs correspond to the expected time intervals.